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- Einstein Chair Mathematics Seminar

# Einstein Chair Mathematics Seminar

**CUNY Graduate Center: 365 Fifth Avenue (between 34th and 35th streets) New York, NY 10016**

The Einstein Chair Mathematics Seminar is concentrated on the relationship between algebraic topology and quantum field theory.

This is like the opposite of JFK's famous saying "Ask not what your country can do for you, but rather ask what you can do for your country".

Namely beyond the noble desire of some mathematicians to clarify the foundations of quantum theory by the definitions and methodology of mathematics specifically to illuminate theoretical physics, there is a slightly different opportunity-mining theoretical physics as it is for the sake of mathematics.

Passing over in silence the commonly held conclusion that quantum field theory is physically successful in that it already contains a viable procedure for making predictions that are verifiable in high energy experiments, one can observe the success of the algorithms associated to the action principle, quantum style, in mathematics itself. There are rigorous and famous discussions in mathematics that are separated in mathematics but are actually unified in the minds of theoretical physicists by these algorithms of quantum field theory.

There are also unsolved areas of mathematics that suffer the same technical difficulties as those in quantum field theory; but while the former field outside of applications seems to languish in the math journals the latter field seems to flourish in the theoretical physics journals.

In the first example one can mention the celebrated invariants of differential topology (Donaldson and Vaughn Jones), of symplectic topology and algebraic geometry {Gromov and Witten), and of complex structures (Kodaira Spencer Griffiths...Kontsevich).

While for the second example one can mention intractable nonlinear PDEs like those appearing in 3D fluid dynamics. One may add in the second example the remark that important examples of tractable nonlinear PDEs, the integrable systems or hierarchies, seem to have a deep connection with quantum field theory and conversely they seem to have a deep role to play there.

There may be several veins of precious math material to mine from this opportunity and several methods.

I am particularly interested in the method of algebraic topology which associates linear objects (homology groups) to nonlinear objects with points ( manifolds...) just like quantum theory associates linear spaces of states to classical systems with points. The main character in algebraic topology is the nilpotent operator or boundary operator while in quantum field theory an important role is played by the nilpotent operators called Q and "delta" which encode whatever symmetry is present in the action of the particular theory and measure the obstruction to invariantly assign meaning to the integral over all paths.

In algebraic topology there is a powerful idea, due first to Stasheff but going beyond his famous and elegant concept of an infinitely homotopy associative algebra, which allows one to live with slightly false algebraic identities in a new world where they become effectively true. In quantum field theory the necessity to regularize or cutoff which sometimes destroys, but only slightly, identities expressing various symmetries and structures may provide an opportunity to use this powerful idea from algebraic topology.

Finally algebraic and geometric topology has always directed it efforts towards understanding in an algebraic way geometric objects like manifolds which are the classical models of spacetime, while quantum field theory often begins its specification of a particular theory with the classical action defined on the classical fields spread over spacetime and then proceeds to its algebraic algorithms.

All these connections suggest that one way to enter the mining business in the above sense is to define relevant algebraic structures with a nilpotent operator and formulate mathematically the intuitively clear physical idea of an "effective theory" as a kind of push forward of the entire algebraic structure in the new world or sense created by the idea underlying Stasheff's famous example of an A-infinity algebra.

The format of the seminar is generous regarding time and allows a robust exchange of information between the expositor and the other participants- who usually ask a lot of questions.

#### ZOOM LOGIN INFORMATION

https://gc-cuny-edu.zoom.us/j/9894931174?pwd=MHprR2Vqa2dvZmVseXAxNzlXZUFhdz09 [gc-cuny-edu.zoom.us]**Meeting ID**: 989 493 1174**Passcode**: einstein

Videos of Einstein Chair seminars from 1981-2014

Youtube website for current videos: www.youtube.com/channel/UC_jFgn51x3iXh8ljGzWRToA

__ Contact Information:__ einsteinchair@gc.cuny.edu

**Date**: March 28, 2023**Speaker**: Stan Srednyak, Duke University**Mode**: In-person**Location**: CUNY Graduate Center, Room 6417

**Date**: February 7, 2023**Speaker**: Prof. Govind Menon, Brown University**Title**: TBA**Mode**: TBA**Time**: 1:45 p.m. - 2:50 p.m.

**Date**: January 31, 2023**Speaker**: Alberto Bressan, Penn State University**Title**: Remarks on the ill-posedness of hyperbolic conservation laws**Mode**: Zoom**Time**: 1:45 p.m. - 2:50 p.m.**Abstract**: For hyperbolic systems of conservation laws in one space dimension,

a general existence-uniqueness theory is now available, which applies to entropy weak solutions with bounded variation. However, as soon as the total variation becomes unbounded, the issue of well-posedness is far from being understood.

The first part of the talk will discuss examples and open questions in the 1-D setting. The second part will focus on the 2-D Euler equations, considering the simplest possible examples of Cauchy problems admitting multiple solutions. Several numerical simulations will be presented, for incompressible as well as compressible flow, indicating the existence of two distinct solutions for the same initial data.

**Date**: January 24, 2023**Speaker**: James Michael Leahy, Imperial College**Mode**: In-person**Location**: CUNY Graduate Center, Room 6417**Title**: Euler-Poincaré Equations (roughly)**Abstract**: Vladimir Arnold’s pioneering work in 1966 clarified that the configuration space for ideal fluids is the Lie group of volume-preserving diffeomorphisms on a given domain and fluid flow is a geodesic on the group with respect to the right invariant Riemannian L2-metric. The system of PDEs that govern the velocity field in ideal fluid flow, discovered by Euler in 1757 are, in fact, the Euler-Poincaré equations, which are the reduced form of the Euler-Lagrange equations discovered by Poincare for a general Lie group and action-invariant Lagrangian in 1901. Moreover, the right-invariance of the metric, equivalent to the so-called particle relabeling symmetry of the kinetic energy, necessitates a conservation law by Noether’s theorem, and this conservation law is the celebrated Kelvin’s circulation theorem. The scope of the geometric and variational approach to fluid dynamics was extended by Holm, Ratiu, and Marsden (1998) to cast many inviscid geophysical fluid dynamic equations as Euler-Poincaré equations on semidirect product Lie algebras by incorporating additional physical variables advected by the flow and adding potential energy terms to the Lagrangian. I will discuss recent joint work with Dan Crisan, Darryl Holm, and Torstein Nilssen, in which we put forward variational principles for fluid dynamics on geometric rough paths by directly working with the reduced form of the Lagrangian and constraining advected variables to be advected by the sum of a smooth and geometric rough-in-time vector field. Critical points are characterized by a system of rough PDEs, which are a perturbed version of the Euler-Poincaré equations derived in Holm, Ratiu, and Marsden (1998). This class of equations can be applied to develop non-Markovian parameterized subgridscale fluid models. In the ideal fluid setting, I will also discuss the well-posedness and blowup criterion for the corresponding system of rough partial differential equations on the torus.

The talk will primarily be based on:

- Crisan, D., Holm, D.D., Leahy, J.M. and Nilssen, T., 2022. Variational principles for fluid dynamics on rough paths. Advances in Mathematics,

404, p.108409. - Crisan, D., Holm, D.D., Leahy, J.M. and Nilssen, T., 2022. Solution properties of the incompressible Euler system with rough path advection.

Journal of Functional Analysis, 283(9), p.109632.

**Date**: November 29, 2022**Speaker**: Robert Palais, University of Utah**Title**: Genomic Medicine From A Mathematician’s Perspective**Abstract**: We will describe technologies and computational methods behind recent advances in DNA analysis, including rapid PCR tests for infectious and genetic diseases, as well as personalized cancer therapies. Ising models, spectral deconvolution, and clustering are some of the tools we use to detect Covid-19, genetic anomalies, and mechanisms of tumor progression. Mathematically informed improvements in the thermodynamic simulations and machine learning algorithms are needed to expand the usefulness of these techniques to diagnose and treat other conditions.**Mode**: In-person**Location**: CUNY Graduate Center, Room 6417 **Time**: 2:00 p.m. - 4:00 p.m.

**Date**: December 7, 2021**Speaker**: Anil Hirani, University of Illinois Urbana-Champaign**Title**: TBA

**Date**: December 1, 2021**Speaker**: Klas Modin, Chalmers University of Technology**Time**: 11:30 a.m. - 1:00 p.m. (EST)**Title**: TBA

**Date**: October 13, 2021**Speaker**: Robert Cardona, UPC Barcelona**Time**: 11:30 a.m. - 1:00 p.m. (EST)**Title**: Universal computation and the Euler equations**Abstract**: In this talk, we review recent results relating computability theory with the Euler equations of hydrodynamics. First, we will introduce Turing machines and the notion of "Turing complete" dynamical system. Then, we present the construction of a Turing complete steady Euler flow on a Riemannian sphere of dimension 3 based on joint work with E. Miranda, D. Peralta-Salas, and F. Presas. The proof combines the Beltrami-Reeb correspondence established by Etnyre and Ghrist and Moore's generalized shifts. Turing complete dynamical systems possess undecidable trajectories, therefore our construction shows that uncomputable phenomena occur in 3D Euler flows. Variations of the construction yield Euler/Reeb flows with other undecidable properties, such as determining whether an explicit orbit is closed.

Time permitting, we will discuss more results in this direction: the Turing completeness of the time-dependent Euler equations on high dimensional manifolds and generalizations for steady solutions on the Euclidean three-dimensional space. Those are joint works with E. Miranda and D. Peralta-Salas.

**Date**: September 28, 2021**Time**: 6:30 p.m. - 8:30 p.m.**Speaker**: Anton Izosimov, University of Arizona**Title**: Lie groupoids in fluid dynamics**Abstract**: In 1966 V. Arnold showed that the motions of an inviscid incompressible fluid (governed by the Euler equation) can be regarded as geodesics on the group of volume-preserving diffeomorphisms, thus making it possible to study hydrodynamics in the framework of infinite-dimensional geometry. In the subsequent years it was shown that Arnold's description also applies in many other fluid-dynamical settings, including magnetohydrodynamics, compressible fluids, etc. However, the scope of applicability of Arnold’s approach is limited to systems whose symmetries form a group. At the same time, there are many problems in fluid dynamics, such as free boundary problems or discontinuous fluid flows with vortex sheets, whose symmetries should instead be regarded as a groupoid.

In the talk, I will outline an extension of Arnold’s framework from groups to groupoids, with vortex sheet motion being the main example. The talk is based on joint work with B. Khesin.

**September** 14, 2021**Time**: 6:30 p.m. - 8:00 p.m.**Speaker**: James Glimm, Stony Brook University**Title**: Construction of nontrivial quantum gauge theories and nontrivial stochastic dynamics: The continuum theory in a finite volume**Abstract**: We prove convergence of renormalized perturbation theory in a finite space time volume for a number of theories in 4 dimensional space time. The proofs are based on generalizations of the Bell number analysis of Yang-Mills Lagrangians. The construction starts in a double cutoff manner, with the finite space time volume and with a discrete lattice formulation. The lattice formulation introduces a problem, in that the nominally gauge invariant Lagrangian, due to its definition on a lattice, involves products of field operators at distinct points. To overcome this obstacle, which can be traced to the well known infinities of these theories, we use the Bell number type analysis only for more highly subtracted quantities, such as the charge, which are ultraviolet convergent, and thus allow evaluation of all quantities in the Lagrangian at a single point.

There are two distinct renormalization methods, which differ in their starting point. They define distinct physical theories, the classical mechanics of stochastic phenomena (e.g. turbulence) and true quantum field theory. Our proof of converged renormalization theory applies to both cases. The theories have distinct equations of motion. For the classical stochastic theory, we recover the Euler and Navier-Stokes equations of fluid turbulence. The equations of motion for the gauge field are the equations of motion for the transport of enstrophy.

August 31, 2021

Time: 6:30 p.m. - 8:00 p.m.

Speaker: Stephen Preston, CUNY Brooklyn College

Title: Breakdown of the mu-Camassa-Holm equation

Abstract: The PDE $m_t + u m_x + 2 u_x m = 0 $, with $ m = \mu – u_{xx} $, where $\mu$ is the average value of $u$

on the circle, is a variation of the Camassa-Holm equation proposed in a paper by Khesin, Lenells, and Misiolek, called the $\mu$-Camassa-Holm equation. It represents geodesics on the diffeomorphism group of the circle. Using a new geometric interpretation, I will show how to prove the breakdown result for C^2 initial data: the solution exists for all time if and only if the initial momentum $m(0,x)$ does not change sign on the circle.

August 17, 2021

Time: 5:00 p.m. - 7:00 p.m.

Speaker: Sasha Migdal, NYU

Title: Vortex Sheets and Turbulent Statistics

Abstract: We review and advance further the vortex sheet theory. The origin of irreversibility is the microscopic stability of the vortex sheet, leading to new boundary conditions (CVS) for the local strain at the surface. We present the exact solutions of the stationary vortex sheet equations and develop the turbulent statistics. The origin of the turbulent statistics is the accumulation of contributions of remote vortex structures to the background strain for each structure. Constant random traceless tensor strain obeys the random matrix distribution. We find a universal asymmetric distribution for energy dissipation. An unexpected new phenomenon is a multifractal distribution of the shape of the cross-section of the vortex tube, leading to the calculable formula for the moments of the velocity field.

August 10, 2021

Time: 2:30 p.m. - 4:30 p.m.

Speaker: Min-Chul Lee, Univ of Oxford

Title: Equivalence Between Euclidean and Minkowski Field Theories

Abstract: For the first half for this talk, we will continue our introduction of QFT and discuss additional details regarding the Reconstruction Theorem. The second half of this talk will the discuss the possibility of a gauge theory for the Navier-Stokes Equations.

August 3, 2021

Time: 2:30 p.m. - 4:30 p.m.

Speaker: Min-Chul Lee, Univ of Oxford

Title: Quantum Field Theory - Basic Mathematical Approach

Abstract: We will present a quick introduction of QFT based on the book "QFT: A Tourist's Guide for Mathematicians" by Folland.

July 20, 2021

Time: 2:30 p.m. - 4:30 p.m.

Speaker: James Glimm, Stony Brook Univ.

Title: Renormalized perturbation theory for the Euler and Navier-Stokes equations

July 13, 2021

Time: 2:30 p.m. - 4:30 p.m.

Speaker: James Glimm, Stony Brook Univ

Title: Quantum Field theory: a sub problem for fluid turbulence

June 29, 2021

Time: 2:30 p.m. - 4:30 p.m.

Speaker: Aseel Farhat, Univ of Virginia

Title: Geometry of 3D NSE and the regularity problem

Abstract: Computational simulations of turbulent flows indicate that the regions of low dissipation feature high degree of local alignment between the velocity and the vorticity. Hence, one could envision a geometric scenario in which the persistence of the local near-Beltrami property might be consistent with a (possible) finite-time singularity formation. We will show that this scenario is in fact prohibited if the sine of the angle between the velocity and the vorticity is small enough with respect to the local enstrophy.

June 15, 2021

Time: 3:30 p.m. - 5:00 p.m.

Speaker: Theo Drivas, Princeton Univ

Title: Discussion of "Finite Time Analyticity for Two and Three-Dimensional Kelvin-Helmholtz Instability" by Sulem-Sulem-Bardos-Frisch, Commun. Math. Phys., 1981.

5:00 p.m. - 6:00 p.m., Sasha Migdal, NYU

Title: Confined Vortex Surfaces and Irreversibility. plus an exact solution for the flow, Part II

Abstract: We continue the study of Confined Vortex Surfaces (CVS) that we introduced in the previous paper. We classify the solutions of the CVS equation and find the analytical formula for the velocity field for arbitrary background strain eigenvalues in the stable region. The vortex surface cross-section has the form of four symmetric hyperbolic sheets with a simple equation |y||x|μ=1 in each quadrant of the tube cross-section (xy plane). We use the dilute gas approximation for the vorticity structures in a turbulent flow, assuming their size is much smaller than the mean distance between them. We vindicate this assumption by the scaling laws for the surface shrinking to zero in the extreme turbulent limit. We introduce the Gaussian random background strain for each vortex surface as an accumulation of a large number of small random contributions coming from other surfaces far away. We compute this self-consistent background strain, relating the variance of the strain to the energy dissipation rate. We find a universal asymmetric distribution for energy dissipation. A new phenomenon is a probability distribution of the shape of the profile of the vortex tube in the xy plane. Its fractal dimension is a random variable distributed between $\ot$ and . This phenomenon naturally leads to the "multi-fractal" scaling of the moments of velocity difference v(r_1)−v(r_2). We argue that the approximate relations for these moments suggested in a recent paper by Sreenivasan and Yakhot are consistent with the \CVS{} theory. We reinterpret their renormalization parameter α≈0.95 in the Bernoulli law p=−12α|v|^2 as a probability to find no vortex surface at a random point in space

Date: June 8, 2021

Time: 2:30 p.m. - 4:30 p.m.

Open meeting for discussion

Date: June 1, 2021

Time: 2:30 p.m.

Speaker: Sasha Migdal, NYU

Title: Confined Vortex Surface and Irreversibility.plus an Exact solution for the flow

Abstract: We revise the steady vortex surface theory following the recent finding of asymmetric vortex sheets (AM,2021). These surfaces avoid the \KH{} instability by adjusting their discontinuity and shape. The vorticity collapses to the sheet only in an exceptional case considered long ago by Burgers and Townsend, where it decays as a Gaussian on both sides of the sheet. In generic asymmetric vortex sheets (Shariff,2021), vorticity leaks to one side or another, making such sheets inadequate for vortex sheet statistics and anomalous dissipation.

We conjecture that the vorticity in a turbulent flow collapses on the special kind of surface. The eigenvalue of the strain tensor along the velocity gap vanishes at this surface. In this case, the Euler vortex sheet solution matches the planar Burgers-Townsend solution of the Navier-Stokes equation in the local boundary layer; otherwise, it matches the asymmetric vortex sheet solution which leaks vorticity.

The most important qualitative observation is that the inequality needed for this solution's stability breaks the Euler dynamics' time reversibility. We interpret this as dynamic irreversibility. We have also represented the enstrophy by a surface integral, conserved in the vortex sheet dynamics in the turbulent limit.

We have found the exact analytic solution for the cylindrical vortex surface for an arbitrary constant background strain with two different eigenvalues. This solution is expressed as a parametric representation using two conformal maps: from the unit circle into the x+iy complex plane a with velocity field v/x + iv/y. We represent these conformal maps as integrals of certain algebraic functions over the unit circle. We have investigated this solution numerically in great detail.

In a coming paper we study turbulent statistics corresponding to this family of steady solutions.

Date: May 25, 2021

Time: 5:30 p.m.

Speaker: Aseel Farhat., Florida State

Title: Geometry of Turbulent Flows and the 3D Navier-Stokes regularity problem

Abstract: We describe several aspects of an analytic/geometric framework for the three-dimensional Navier-Stokes regularity problem, which is directly inspired by the morphology of the regions of intense vorticity/velocity gradients observed in computational simulations of three-dimensional turbulence. Among these, we present our proof that the scaling gap in the 3D Navier-Stokes regularity problem can be reduced by an algebraic factor within an appropriate functional setting incorporating the intermittency of the spatial regions of high vorticity. We will also show that that 3D NSE are regular under some appropriate local condition on the helicity in the the regions of intense vorticity.

Date: May 18, 2021

Time: 5:30 p.m.

Speaker: Theo Drivas, Stony Brook University

Title: Part II On the mystery of large vortex patterns in long time 2D ideal euler fluid motion

Abstract: We will discuss some aspects of inviscid, incompressible (perfect) fluid motion in two dimensions.

We describe some old and new results and open questions regarding properties of steady solutions of

the two-dimensional incompressible Euler equations, as well as properties of nearby trajectories. Specifically,

we focus on whether steady states can be isolated, whether, for solutions starting nearby steady states, recurrence can occur and whether singularities must form at long times. Finally, we discuss the infinite-time limit near and far from equilibrium with a focus on the apparent irreversibility/decrease of entropy. See related movies:

https://www.youtube.com/watch?v=25Md9qxIReE [youtube.com]

https://www.youtube.com/watch?v=-YdEYumSSJ0&t=222s [youtube.com]

https://www.youtube.com/watch?v=a3ENdIy1WL0 [youtube.com]

Date: May 11, 2021

Time: 5:30 p.m.

Speaker: Theo Drivas, Stony Brook

Title: Asymptotic pictures of and for 2D incompressible fluid motion without friction

Abstract: We will discuss some aspects of inviscid, incompressible (perfect) fluid motion in two dimensions.

We describe some old and new results and open questions regarding properties of steady solutions of

the two-dimensional incompressible Euler equations, as well as properties of nearby trajectories. Specifically,

we focus on whether steady states can be isolated, whether, for solutions starting nearby steady states, recurrence can occur and whether singularities must form at long times. Finally, we discuss the infinite-time limit near and far from equilibrium with a focus on the apparent irreversibility/decrease of entropy. See related movies:

https://www.youtube.com/watch?v=25Md9qxIReE [youtube.com]

https://www.youtube.com/watch?v=-YdEYumSSJ0&t=222s [youtube.com]

https://www.youtube.com/watch?v=a3ENdIy1WL0 [youtube.com]

Date: May 4, 2021

Time: 5:30 p.m

Speaker: V. Martinez, Hunter College

Title: KDV, uniqueness of invariant measures and beyond

Abstract: We discuss the existence, uniqueness, and regularity of invariant measures for the damped-driven stochastic Korteweg-de Vries equation, where the noise is additive and sufficiently non-degenerate. It is shown that a simple, but versatile control strategy, typically employed to establish exponential mixing for strongly dissipative systems such as the 2D Navier-Stokes equations, can nevertheless be applied in this weakly dissipative setting to establish both unique ergodicity, albeit without mixing rates, as well as regularity of the support of the invariant measure. Under the assumption of large damping, however, exponential mixing can be recovered.

Date: April 27, 2021

Time: Fluids seminar, 530 to 700

Speaker: Ali Khosronejad

Title: High-fidelity numerical simulation of turbulent fluid flows in arbitrarily complex domains

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Abstract: I present in this talk an overview of my research focus on developing three-phase multi-scale multi-physics computational fluid dynamics model for a wide range of applications such as renewable energy, biological flows, and transport processes in the atmosphere and riverine systems. I also present the simulation results obtained from the large-eddy simulation method of turbulent flows across different scales -- from micros (human saliva particles) to mesoscale (atmospheric boundary layer in offshore wind farms).

Date: April 20, 2021

Time: 5:30 p.m.

Speaker: Open meeting for discussion

Date: Tuesday, April 13, 2021

Time: 5:30 p.m.

Speaker: Vincent Martinez, Hunter College

Title: More on KdV and invariant measures

Abstract: We discuss the existence, uniqueness, and regularity of invariant measures for the damped-driven stochastic Korteweg-de Vries equation, where the noise is additive and sufficiently non-degenerate. It is shown that a simple, but versatile control strategy, typically employed to establish exponential mixing for strongly dissipative systems such as the 2D Navier-Stokes equations, can nevertheless be applied in this weakly dissipative setting to establish both unique ergodicity, albeit without mixing rates, as well as regularity of the support of the invariant measure. Under the assumption of large damping, however, exponential mixing can be recovered.

Date: Tuesday, April 6, 2021

Time: 5:30 p.m.

Speaker: Jim Glimm, Stony Brook University

Title: Turbulence models and quantum ideas

NOTE: THERE ARE 2 TALKS ON MARCH 30, 2021

Date: Tuesday, March 30, 2021

Time: 1:30 p.m. to 2:30 p.m.

Speaker: Gregory Falkovich, Weizmann Institute

Title: Fibonacci Turbulence

Abstract: Never is the difference between thermal equilibrium and turbulence so dramatic, as when a quadratic invariant makes the equilibrium statistics exactly Gaussian with independently fluctuating modes. That happens in two very different yet deeply connected classes of systems: incompressible hydrodynamics and resonantly interacting waves. I shall describe an information-theoretic analysis of turbulence in such strongly interacting systems. The analysis involves both energy and entropy and elucidates the fundamental roles of space and time in setting the cascade direction and the changes of the statistics along it. We introduce a beautifully simple yet rich family of discrete models (sets of ODE) with triplet interactions of neighboring modes and show that it has quadratic conservation laws defined by the Fibonacci numbers. Depending on how the interaction time changes with the mode number, three types of turbulence were found: single direct cascade, double cascade, and the first ever case of a single inverse cascade. We describe quantitatively how deviation from thermal equilibrium all the way to turbulent cascades makes statistics increasingly non-Gaussian and find the self-similar form of the one-mode probability distribution. I will end up discussing possible directions of an analytic theory including applications of Renormalization Group to this class of models.

Time: 5:30 p.m. to 6:30 p.m.

Speaker: Vincent Martinez, Hunter College

Title: "On unique ergodicity and mixing for the damped-driven stochastic KdV equation"

Abstract: We discuss the existence, uniqueness, and regularity of invariant measures for the damped-driven stochastic Korteweg-de Vries equation, where the noise is additive and sufficiently non-degenerate. It is shown that a simple, but versatile control strategy, typically employed to establish exponential mixing for strongly dissipative systems such as the 2D Navier-Stokes equations, can nevertheless be applied in this weakly dissipative setting to establish both unique ergodicity, albeit without mixing rates, as well as regularity of the support of the invariant measure. Under the assumption of large damping, however, exponential mixing can be recovered.

Date: Tuesday, March 16, 2021

Time: 5:30 p.m.

Speaker: Tarek Elgindi, Duke University

Title: "Stationary Incompressible ideal fluid motions in two dimensions"

Dates: March 8-12, 2021

Einstein Chair seminar replaced by: Virtual Workshop Many faces of renormalization: March 8-12, 2021

http://scgp.stonybrook.edu/archives/30366 [scgp.stonybrook.edu]

Date: March 2, 2021 (Tuesday)

Time: 5:30 p.m. - 6:30 p.m.

Speaker: Theodore Drivas, Princeton Univ.

Title: A Lagrangian perspective on anomalous dissipation.

Abstract: We will review some observations and results that connect (near)

random motion of tracer particles in (near) rough velocities fields. In fluid turbulence,

this connection has been known to exist since the pioneering work of L.F. Richardson.

For passive scalar fields (non-reactive dye), it was observed by Bernard-Gawedzki-Kupiainen

in the context of the Kraichnan model that the stochastic behavior of particles is explicitly

connected to anomalous dissipation of scalar fluctuations in the non-diffusive limit. A similar

statement can be made for a pressureless fluid in one-dimension which loses energy at shocks.

We will end with some speculative remarks about incompressible Navier-Stokes turbulence.

ZOOM LOGIN INFORMATION:

https://gc-cuny-edu.zoom.us/j/9894931174?pwd=MHprR2Vqa2dvZmVseXAxNzlXZUFhdz09 [gc-cuny-edu.zoom.us]

Meeting ID: 989 493 1174

Passcode: einstein

Date: February 8, 2021

Time: 2:00 p.m. - 3:30 p.m.; discussion until 4:00 p.m.

Speaker: Theodore Drivas, Princeton university

Title: Remarks on mathematical foundations of Kolmogorov's 1941 turbulence theory

Abstract: In this talk, we review certain aspects of three-dimensional incompressible

fluid turbulence, focusing on the phenomenon of anomalous dissipation. We will describe

in detail Duchon-Robert's (2000) contribution to establishing a rigorous framework with which to

discuss anomalous energy flux due to possible singularities in Leray solutions and, in the inviscid limit,

singular weak solutions of the Euler equations in the inviscid limit as envisioned by Onsager.

ZOOM LOGIN INFORMATION:

https://gc-cuny-edu.zoom.us/j/9894931174?pwd=MHprR2Vqa2dvZmVseXAxNzlXZUFhdz09 [gc-cuny-edu.zoom.us]

Meeting ID: 989 493 1174

Passcode: einstein

Date: February 1, 2021

Time: 2:00 p.m. - 3:30 p.m.; discussion until 4:00 p.m.

Speaker: Daniel An, SUNY Maritime College

Title: A report on lattice fluid model simulations

Date: January 25, 2021

Time: 2:00 p.m. - 3:30 p.m.; discussion until 4:00 p.m.

Speaker: Tarek Elgindi, Theo Drivas and Dennis Sullivan

Informal discussion

Date: January 18, 2021

Time: 2:00 p.m. - 3:30 p.m.; discussion until 4:00 p.m.

Speaker: Tarek Elgindi, UC San Diego

Title: Holder continuous initial vorticity developing singulatity in finite time for Euler

Date: January 11, 2021

Time: 2:00 p.m. - 3:30 p.m. discussion until 4:00 p.m.

Speaker: Pooja Rao, Stony Brook University

Title: Part II of layer instabilities and front tracking

(these two lectures from PhD work with J.Glimm)

Date: January 4, 2021

Time: 2:00 p.m. - 3:30 p.m.; discussion until 4:00 p.m.

Speaker: Abigail Hsu, Stony Brook University

Title: Probing the statistical structure using direct numerical simulation of turbulence

Abstract: Examine the intermittency phenomenon that signifies coexistence of turbulent regions with regions is still an open question in modern turbulence theory. One way to examine this phenomenon, is to study statistical properties of the scalar increment, i.e. the difference of the scalar field at two points separated by a distance l. We are interested in developing a universal scaling law applicable to velocity increments and velocity gradient as these quantities vary across length scales. In the localized ranges of length scales in which the turbulence is only partially developed, we propose multifractal scaling laws with scaling exponents modified from the classical inertial range values. We have discovered the deviation from the classical theories for the energy dissipation rate in the inertial range. The scaling exponent of the velocity increment zeta_p and the exponent of the energy dissipation rate tau_p for the p-th moment structure functions are linear in log length in the dissipation range. New parameterized models have been proposed with explicit formulas that characterize this relation for high order moment statistics. DNS data verifications subsume and extend the existing models of inertial range turbulence and are especially focused on the dissipation range for small length scales. This study finds quantitative corrections to the theory of fully developed turbulence which describes partially developed turbulent flows.

Date: December 28, 2020

Time: 2-3:30p; discussion until 4pm

Speaker: Dennis Sullivan, Stony Brook Univ/CUNY Grad Center

Title: Perturbative expansion of RHS of effective fluid ODEs based on Algebra of NaCl lattice coarse graining of the continuum

Date: December 21, 2020

Time: 2-3:30p; discussion until 4pm

Speaker: Dennis Sullivan, Stony Brook Univ/CUNY Grad Center

Title: Perturbative expansion of RHS of effective fluid ODEs based on Algebra of NaCl lattice coarse graining of the continuum

Date: December 14, 2020

Time: 2-3:30pm; discussion until 4pm

Speaker: Stephen Preston, Brooklyn college/ CUNY Grad center

Title: Euler equations as geodesics on diffeomorphism groups

Abstract: I will discuss Arnold’s perspective on the Euler equations as geodesics on the group of volume-preserving diffeomorphisms, with an eye to generalizations on other diffeomorphism groups. I’ll talk about the general notion of vorticity conservation and what it looks like in 2D, 3D, and in the 3D axisymmetric case. I will also describe how this infinite-dimensional geometry approach allows for simpler proofs of the local existence result. In addition we’ll look at curvature computations and what they say about stability of fluids, along with conjugate points along geodesics and how they differ in 2D and 3D.

In the second half of the talk I will describe other PDEs that can be described as geodesics on diffeomorphism groups, including 1D models that can be understood more easily, including the breakdown mechanisms and the global existence of weak solutions.

Date: December 7, 2020

Time: 2-3:30p; Discussion until 4pm

Speaker: Pooja Rao, Stony Brook University & Theo Drivas, Princeton University

Title: Modeling mixing in interfacial instabilities

Abstract: Turbulent mixing from hydrodynamic instabilities, such as the Richtmyer-Meshkov (RMI) and Rayleigh-Taylor (RTI) instabilities, plays a critical role in numerous applications ranging from performance degradation in inertial confinement fusion capsules to supernova explosions. To accurately model these flow regimes requires special treatment of the interface between the two fluids. In this talk, we introduce one such numerical approach, Front-tracking, which has shown great success in modeling interfacial instabilities. To build up this approach, we first introduce the basics of these interfacial instabilities and discuss the main theoretical ideas behind a Riemann problem and the Front-tracking algorithm in 1-dimension. Next, we build on these ideas and show some comparisons between Front-tracking and other approaches. This is followed by reviewing results from simulations of the Richtmyer-Meshkov instability in a 2-dimensional setting using our proposed framework.

Date: November 30, 2020

Time: 2-3:30p; Discussion until 4pm

Speaker: Stephen Preston, Brooklyn college/ CUNY Grad Center

Title: “Euler equations as geodesics on diffeomorphism groups”

Abstract: I will discuss Arnold’s perspective on the Euler equations as geodesics on the group of volume-preserving diffeomorphisms, with an eye to generalizations on other diffeomorphism groups. I’ll talk about the general notion of vorticity conservation and what it looks like in 2D, 3D, and in the 3D axisymmetric case. I will also describe how this infinite-dimensional geometry approach allows for simpler proofs of the local existence result. In addition we’ll look at curvature computations and what they say about stability of fluids, along with conjugate points along geodesics and how they differ in 2D and 3D.

In the second half of the talk I will describe other PDEs that can be described as geodesics on diffeomorphism groups, including 1D models that can be understood more easily, including the breakdown mechanisms and the global existence of weak solutions.

Date: Nov 23, 2020

Time: 2-3:30p; Discussion until 4pm

Speaker: Dennis Sullivan, Stony Brook Univ/ CUNY Grad Center

Title: Properties of the overlapping decompositions coarse graining the continuum

Abstract: The primary vorticity model for 3D fluids is described in terms of the h- lattice vector calculus in 3D made out of 8 overlapping cubical grids of edge size 2h obtained by all shifts by multiples of h.The goal is to describe secondary, tertiary, ... vorticity models systematically based on the higher deviations from the leibniz rule. The kth deviation for each k, a multilinear operator with k+1 arguments, is shown to be divisible by the k-1st power of h, the scale regarded as a formal variable.

Date: Nov 16, 2020

Time: 2-3:30p; Discussion until 4pm

Speaker: Theodore Drivas princeton university

Title: Some remarks on lattice hydrodynamics and consistency with the Navier-Stokes description.

Abstract: We discuss a class of lattice hydrodynamical models introduced recently by Dennis

Sullivan and collaborators. After reviewing some elementary properties of these models

(precise formulation, conservation laws), we will outline an argument to prove consistency

with the continuum Navier-Stokes picture in the regime where the lattice is sufficiently fine

and Navier-Stokes solution is well behaved. A biproduct of the proof is that, for any interval

of existence/uniqueness of Navier-Stokes, the lattice models admit solutions provided again

that the grid is fine enough. The proof is quantitative, with convergence rates depending only

on norms of the continuum Navier-Stokes solutions.

Date: November 2, 2020

Time: 2-3:30p; Discussion until 4pm

Speaker: Dennis Sullivan, Stony Brook Univ/CUNY Grad Center

Title: Finite approximations to vector calculus in 3D and Leibniz deviations

Abtract: Creating the star operator leads to an overlapping cubical system whose chains and cochains are direct sums over the lattice points of Grassmann algebras. It consists of a standard scale h cubical decompositions and their shifts by half integer multiples of the scale. This gives in 3D eight overlapping cubical grids.

The boundary and coboundary are conjugate by the poincare dual operator which is the star operator.

In a paper to appear in the Atiyah Memorial volume (QJM) with Nissim Ranade and Ruth Lawrence the Leibniz deviations are described and are the same as the commutators discussed by C. Braun and A. Lazarev which denote this structure a commutative BV-infinity algebra in their arxiv 2013 paper.

Adding the scale h as a formal parameter these commutators and the higher cumulants of natural mappings between scales for these lattice examples are shown in the Atiyah paper to be divisible by appropriate powers of h for perturbative QFT as discussed in arxiv 2018 by Jae Suk Park. The Leibniz deviations turn out to be natural statistically as well.

It would be interesting to compare this structure with the rigorous discussion in the book by Kevin Costello 2012 on Renormalization and Effective Field Theory in the BV formalism of the continuum.

Date: October 19, 2020

Time: 2-3:30pm; Discussion until 4pm

Speaker: Dennis Sullivan, Stony Brook Univ/CUNY Grad Center

Title : The role of Leibniz rule in 3D fluid computation I

Abstract: Finite differences do not satisfy the leibniz rule for the product. This affects the behaviour of computations for fluid dynamics.

We will describe the traditional role of the leibniz rule in combinatorial topology. Then its consequences for discretizations related to coding and computation.

Finally, the paradox that actual computations unearthed with collaborators Daniel An and Alice Kwon.

Date: March 2, 2020

Speaker: Theodore Drivas, Princeton University

Title: Flexibility and rigidity and steady fluid motion

Abstract: We discuss the possibility and the impossibility of smooth deformations of equilibrium fluid states in the presence of symmetries.

Date: February 24, 2020

Speaker: Francisco Javier Torres de Lizaur

Title: Dynamics of steady state solutions of the Euler equations I and II

Abstract: In the first lecture, after some introductory remarks on the motion of ideal fluid flows, I will show how to characterize those volume-preserving vector fields on a manifold that are steady solutions of the Euler equation for some Riemannian metric (joint work with Daniel Peralta-Salas and Ana Rechtman). For a given vector field, the existence of such a metric depends on the existence of a limit to the precision with which its asymptotic cycles can be approximated by certain classes of loops. The result extends the characterization of geodesible flows in the volume preserving case (Sullivan). Some applications of this characterization will then be presented. For example, I will give a description of the set of possible vorticities of a given vector field, that is, the set of vector fields that can be written as the curl of the given one for some metric on the manifold; this provides in particular a characterization of Reeb vector fields of contact structures.

In the second lecture, we will discuss more generally what arrangements of trajectories can and cannot be found in a steady Euler flow, and some open problems. We will also report on the recent construction by R. Cardona, D. Peralta-Salas, E. Miranda and F. Presas of steady Euler flows in high dimensional spheres that posses trajectories whose eventual transit over a certain open set is equivalent to the halt of any given Turing machine.

Date: February 10, 2020

10:00-11:30a

A Universal HKR Theorem

Tasos Molinos

The Hochschild-Konstant-Rosenberg theorem is a classic result identifying the Hochschild ho- mology of a commutative ring with differential forms. In characteristic zero, this can be promoted to an equivalence at the level of differential graded algebras, giving rise to the Hodge decomposition on Hochschild homology. Moreover, via this description, one interprets the de Rham differential on differential forms as the natural S1-action on Hochschild homology. In fact, these construc- tions globalize, and so one obtains an equivalence of the derived loop space LX (whose algebra of functions is Hochschild homology) with the shifted tangent bundle T[−1]X = Spec(Sym(ΩX [1]))

In this talk, I will explain how this fails in nonzero characteristic and will describe the discrep- ancy in the form of a natural filtration on the Hochschild homology complex, whose associated

graded is derived de Rham cohomology. In characteristic zero, this filtration

naturally splits. I will review the relevant notions from derived algebraic

geometry used to construct this filtration before explaining the construction

in greater depth. Time permitting, I will discuss several applications of this

construction and its variants, to a notion of shifted symplectic structures in

characteristic p, and to a potentially very clean conceptual reinterpretation

of the Witten genus in the algebro-geometric setting. Much of the above is

joint work with Marco Robalo and Bertrand Töen.

2:30-3:30p

Dev Sinha

Finite-type knot invariants and the embedding calculus (I)

We share progress on the a program to prove that approximations, which capture the homotopy class of the map on the configuration spaces of distinct points on the circle to distinct points in three space induced by a knot, serves as a universal invariant over the integers.(goodwillie-weiss &vassiliev). Such a

result generalizing the linking number integral of gauss was essentially conjectured earlier in the discussion of integrals associated to a quantum theory (bott & taubes). Currently such an invariant over the integers is only known to exist in a formal sense. Over the real numbers, such an invariant was constructed using ideas from field theory (kontsevich) so constructing such an invariant homotopically is very much in line with the aims of the einstein chair seminar online blurb.

In our first lecture we revisit the homology and cohomology of configuration

spaces, both for calculations for induced maps and as, surprisingly, the

cohomology gives a presentation for Lie co-algebras useful in

rational homotopy theory. We use this presentation to revisit the

homotopy periods problem, showing that an appropriate construction

(dan quillen) governs higher linking invariants for homotopy groups which we

call Hopf invariants. These integrals which arise as homotopy periods

also arise in field theory.

TEA

4:00-5:00p

Dev Sinha

Finite-type knot invariants and the embedding calculus (II)

We start the second lecture by giving two models for the approximations, one through punctured knots and another through configuration spaces. We review progress on the main conjecture, first recalling the result that related approximations constitute a universal invariant over the real numbers,( volic) through importing configuration space integrals, then discussing the degree two case of the conjecture over the integers, and then discussing recent work:

- construction of a relevant spectral sequence at the level of components

- use of Hopf invariants to give a new construction of the iterated integral invariants (chen -milnor)

- the result using gropes and claspers to resolve the conjecture on component surjectivity ( current bonn thesis of kosanovic)

Time permitting, we can review other embedding and diffeomorphism results and the conjectural picture that the resolution of singularities (vassiliev) seems to give homology, claspers seem to give homotopy, configuration space integrals seem to give de Rham cohomology, and the goodwillie-weiss towers govern all of these structures.

Date: October 21, 2019

Speaker: Prof. Misha Gromov, IHES

2:30 p.m. - (Background lecture) Title: Mathematical beauty of Life

Abstract: Mathematical beauty of Life -- What is this? In fact: What is "Life"? What is "beauty"? What is "mathematics"?

"Life" means the "structure of live entities" the beauty of which unravels on subcellular and molecular levels.

"Beauty" is a psychological phenomenon with a nowhere written ten-page description in mathematical terms.

"Mathematics" is a concept that defies a meaningful definition.

"Mathematical beauty" is whichever may entertain a mathematician's brain.

I start with a few examples where mathematics was ready for reflection of this beauty and

in the second lecture I will show the limitation of our mathematics when it comes the essential structures on which the life is based upon, such as proteins.

< Tea 3:30p>

4:15p-(Main lecture) Title: Protein Spaces

Date: March 11, 2019

Speaker: Prof. Dennis Sullivan (joint work with Alice Kwon)

Title: Application of Perelman Theorem: All Closed Three Manifolds Have Schottky Presentations

Abstract: From the eight lie groups appearing in the Perelman Theorem, one explicitly constructs five lie group actions on the three sphere (only one is pesky). These generate a group G5 action on the three sphere where each element acts by a transformation which is real analytic outside a finite set.

For each closed oriented three manifold M one constructs a discrete subgroup of G5 which has a dense domain of discontinuity U on S3 with complement a totally disconnected limit set L so that U mod the discrete group is M. This is the Schottky presentation of the title. For a given M all Schottky presentations are topologically conjugate.

The construction depends on finitely many parameters that resemble usual Teichmuller spaces and a new analogue. The core of the construction is a G5 structure on M, namely a cover of M by coordinates in S3 whose transitions are performed by elements from G5 that generate the Schottky presentation.

Note: background material will be presented in Sullivan class/seminar 11:00 to 1:00.

March 11, same location.

Date: March 4, 2019-CANCELLED BECAUSE OF SNOW DAY.

Speaker: Dennis Sullivan (joint work with Alice Kwon)

Title: Application of Perelman Theorem: All Closed Three Manifolds Have Schottky Presentations

Abstract: From the eight lie groups appearing in the Perelman Theorem, one explicitly constructs five lie group actions on the three sphere (only one is pesky). These generate a group G5 where each element acts by a transformation real analytic outside a finite set.

For each closed oriented three manifold M one constructs a discrete subgroup of G5 which has a dense domain of discontinuity U on S3 with complement a totally disconnected limit set L so that U mod the discrete group is M. This is the Schottky presentation of the title.

The construction depends on finitely many parameters that resemble usual Teichmuller space and a new analogue. The core of the construction is a G5 structure on M, namely a cover of M by coordinates in S3 whose transitions are performed by elements from G5 that generate the Schottky presentation.

Date: February 11, 2019

Speaker: Prof. Andre Joyal, Université du Québec à Montréal

First talk: (more introductory part of the abstract).

10:45a-12:45p

"Duality in (higher) topos theory"

We claim that topos theory is best understood from a dual algebraic point of view. We shall use the term *logos* for the notion of topos dualized. A logos is a ring-like structure and the theory of logoi has many things in common with the theory of commutative rings. The free logos on one generator Set[X] can be described explicitly. The 2-category of topoi is defined to be the opposite of that of logoi. The topos-logos duality is closely related to the locale-frame duality, and to the duality between affine schemes and commutative rings. Similar observations apply to higher topoi and higher logoi. Rezk's descent principle plays a central role in the theory of higher logoi. We shall sketch a proof of Rezk's descent principle in the category of spaces (=infinity groupoids). We shall sketch the connection between higher topos theory and Homotopy Type Theory.

Second talk: (independent continuation of abstract)

2:00p-3:30p

"A generalised Blakers-Massey theorem in higher toposes"

We prove a generalization of the classical connectivity theorem of Blakers-Massey, valid in an arbitrary higher topos and with respect to an arbitrary modality, that is, a factorization system (L, R) in which the left class is stable by base change. We rederive the classical result, as well as some recent generalization.

Our proof of BM theorem uses Rezk's descent principle and it is inspired by the proof discovered in Homotopy Type Theory.

Date: February 4, 2019

Speaker: Prof. Christoforos Neofytidis, Universite' de Geneve

Title: Self-homeomorphisms of reducible 3-manifolds and applications in topology, geometry and dynamics

Abstract:

In the first part of this talk, we recall the self-homeomorphisms of a closed oriented reducible 3-manifold M. In the second part, we use this description to understand various problems:

(i) We show that M admits an Anosov torus (i.e. an embedded 2-torus T in M that is invariant under a self-homeomorphism f of M and such that f restricted to T is Anosov) if and only if one of its prime summands admits an Anosov torus. Equivalently, one of the prime summands of M is either the 3-torus, the mapping torus of -id on the 2-torus, or a mapping torus of a hyperbolic automorphism of the 2-torus. This answers a question of F. Rodriquez Hertz, J. Rodriquez Hertz and R. Ures. [Joint work with Shicheng Wang.]

(ii) We show that the mapping torus of any closed 3-manifold has zero simplicial volume. This has various consequences: For example, it implies that all mapping tori have zero simplicial volume only in dimensions two and four. Also, it implies another proof of the fact that the fundamental group G of a mapping torus of a closed 3-manifold N is hyperbolic (if and) only if N is virtually a connected sum of copies of S^2 x S^1 and G does not contain Z^2. [Joint work with Michelle Bucher.]

(iii) (If time permits.) We show that if M contains an aspherical prime summand, then, given any self-homeomomorphism f of M, there is a map of degree one from a finite cover of the mapping torus of f on M to a mapping torus of a certain aspherical 3-manifold. This verifies all cases of a conjecture of Y. Ni and T.-J. Li that any mapping torus of a reducible 3-manifold M not covered by S^2 x S^1 has virtually infinite first Betti number, except when M is virtually a connected sum of copies of S^2 x S^1 and of T^2 bundles. [Remark: Ni verified the conjecture by finding certain surjections on the (weaker) level of fundamental groups.]

First talk: 11:00am to 12:30pm (more introductory part of the abstract).

Second talk: 2:00p to 3:30p (independent continuation of abstract)

TEA

Date: December 17, 2018

Speaker: Dennis Sullivan, Stony Brook/GC

Topic: "Uniformizing Three Manifolds with Rigid Coordinates"

Abstract: Uniformizing an object, like the solution set of an equation, means parametrizing, describing, or labeling the points of the object by points in a standard coordinate space or model space.

Here the object is a closed three manifold which may be described in a very rigid way as having limiting geometries on each piece after cutting along two spheres and tori. The goal then is to use this data to construct a slightly less rigid description of the entire glued up three manifold in terms of coordinate charts in the three sphere = R^3 union {infinity},,,,,,

whose overlap maps are restrictions of rigid homeomorphisms taken from one of five lie groups acting on the three sphere, The inverse maps to the charts assemble* to define an equivariant parametrization of the three manifold by an open dense subset of the three sphere

whose complement is either finite [ if M is not a non trivial connected sum] or a standard cantor set [ if M is a non trivial connected sum].

* joint work with Alice Kwon

Date: December 3, 2018

Speaker: Mahmoud Zeinalian, GC

These two lectures are intended to be introductory and the afternoon session will not assume the material discussed in the morning.

Title: A previously unnoticed fact about the singular chains functor (Monday 11:00a-12:30p and 2:00p-3:30p)

Abstract: The singular chains functor, which has traditionally been treated as a functor (topological spaces, weak homotopy equivalences) ——> (dg coalgebras, quasi-isomorphisms) factors through the natural forgetful functor (dg coalgebras, Koszul quasi-isomorphism) ——> (dg coalgberas, quasi-isomorphism). A map of dg coalgebras is called a Koszul quasi-isomorphism if it induces a quasi-isomorphism after applying cobar. This is a strictly stronger notion of weak equivalence on coalgebras than that of quasi-isomorphisms. We will discuss the meaning and some of the consequences of this fact. For instance, the Koszul quasi-isomorphism class of the E-infinity coalgebra of singular chains on a connected pointed space determines the isomorphism class of the fundamental group of that space and it does so functorially. This is all joint work with Manuel Rivera. The Whitehead theorem statement is a joint work with Manuel Rivera and Felix Wierstra.

Below are some of the concepts we will discuss. All concepts and terminology will be defined. Additional Monday and Wednesday lectures can be scheduled to discuss the material based on demand.

-Modeling the singular chains on the Moore path groupoid using basic category theory (work of André Joyal and Jacob Lurie), -Various notions of nerves of categories and their adjoints, -Bar and cobar constructions, -Extent to which bar and cobar constructions are quasi-isomorphism invariant (bar always and cobar sometimes), -The left adjoint to the dg-nerve functor and its close relation to the cobar construction, -Various notions of weak eq on dg coalgebras (quasi-isomorphism (qi) is one, but there are others eg Kozsul qi which means qi after applying cobar) -Singular chains on a space and Koszul quasi-isomorphism of dg coalgebras, -E-infinity coalgebra structure of the chains, -Hopf algebras e.g. group rings and group-like elements -A group=group-like elements of the group ring of the group (integral domain coefficients needed) -Generalization of Whitehead theorem to non-simply connected spaces: a continuous map between connected topological spaces is a weak homotopy equivalence if and only if it induces a Koszul quasi-isomorphism on the chains (the classical theorem uses quasi-isomorphism but needs a simply connected assumption).

-Three different but equivalent notions of derived families of chain complexes and their derived colimit (assembly)

-Avenues of research:

A) Extend Mandell’s work on the E-infinity structure of chains to the non-simply connected land using (E-infinity coalgebras, Koszul quasi-isomorphisms),

B) Use our above understanding of derived families to better understand Ranicki’s theory of derived families of complexes with duality, their assembly, and the associated bordism theory.

Date: November 26, 2018

Speaker: Dennis Sullivan, Stony Brook University/CUNY Graduate Center

Title: The Structure of the Transition Maps for Manifold objects and their Bundles

Abstract: Smooth manifolds and stable vector bundles map to their piecewise linear analogues and then to their homotopy analogues.

The bundles theories can be described by maps of the universal spaces

BO ----:- BPL BO ---:- BG BPL---;- BG

The first map has a fibre with finite homotopy groups starting in dimension seven. The second map is described completely by the Adams operations.Tthe third map can be described completely at odd primes.

The third map has image and kernel described by the concept of having or not having a KO Thom class. The first map ican be described completely by the KO characteristic classes associated to the adams operations applied to the KO Thom class.

In dimensions 5,6,7, the statements about bundles give analogous definitive statements about simply connected manifold objects in the three categories which can be augmented at the prime two for the third map.

Date: March 26, 2018

Speaker: Ralph Kaufman, Purdue University

Title: Cubical Feynman

Abstract: Feynman categories encode the basic features of higher operations. In one aspect, they encode the combinatorics of moduli spaces. In another they are also the basis for considering structures from quantum field theory. One further aspect are Hopf algebras based on their morphisms.

We will discuss new developments in this framework. One is that using W-constructions and push-forward, one obtains cubical complexes that realize moduli spaces and cubical complexes used in Outer Space and Cutkosky rulesThese constructions work particularly well for cubical Feynman complexes, that is those that basically have quadratic relations.These quadratic relations generalize to quadratic algebras and Hopf algebras in the sense of Manin, which we plan to discuss in the end.

Date: March 5, 2018

Speaker: Alena Erchenko, Penn State

Title: Flexibility questions in dynamical systems and first results

Abstract: We introduce the flexibility program proposed by A. Katok which is a big area that up to now has barely touched and consists of a lot of interesting natural questions. In this talk, we discuss some known results. In particular, we show the flexibility of the entropy with respect to the Liouville measure and topological entropy for geodesic flow on negatively curved surfaces with fixed genus and total area (joint with A. Katok). Also, we point out some restrictions which come from additionally fixed a conformal class of metrics (joing with T. Barthelme'). In both settings, we point out connections with flexibility of geometric data.

Date: February 26, 2018

Speaker: Prof. Graeme Segal, Oxford University

Date: May 1, 2017

Speaker: Curt T. McMullen, Harvard University

Title: Cubic curves and totally geodesic subvarieties of moduli space

Abstract: A subvariety V of the moduli space of Riemann surface M_g is totally geodesic if every

Teichmueller geodesic tangent to V is contained in V. Totally geodesic curves are rare

jewels, connected to billiards with optimal dynamics and Jacobians with real multiplication.

The first such curve was discovered by Veech in the 1980s. It is part of an infinite series,

based on special Euclidean triangles.

In this talk we will discuss the discovery, in 2016, of the first known totally geodesic surface

in moduli space. This discovery was based on an unexpected connection between Teichmueller

theory and the classical algebraic geometry of cubic plane curves. It can now be seen as part of

a suite of 6 examples, based on special Euclidean quadrilaterals.

Date: February 6, 2017

Speaker: Manuel Rivera, CINVESTAV

Title: : How categorical ideas extend a striking classical result for simply connected spaces to general spaces

OR

How to derive homological models of the based loop space of non-simply connected spaces.

Abstract: We will describe a generalization of a celebrated classical result of Frank Adams, where he shows an algebraic construction which is defined for any differential coalgebra [connected in degree zero] and produces a differential algebra [ not necessarily connected in degree zero] models the passage which starts from the chains on a connected space which form a coassociative differential coalgebra and goes to the chains on the model of the based loop space due to John Moore, the latter being a differential graded associative algebra using the loop space multiplication. It was important for Adams that the space in question be simply connected so that the based loop space would be connected as well.

We will show, 50 years later, for ANY pointed, connected topological space (X, b), not necessarily simply connected, this SAME algebraic construction [which Adams termed the COBAR construction] applied to the differential graded coalgebra of normalized singular chains in X with vertices at b is homologically equivalent via (dga) maps to the differential graded associative algebra of singular chains on the Moore based loop space of X at b. In this statement we may replace singular chains in X with a homologically equivalent combinatorial model such as the combinatorially object constructed by Dan Kan. We will deduce this result from more general categorical results which are of independent interest.

Namely, the lecture will describe a cubical interpretation of the “rigidification" functor (originally defined by Jacob Lurie) from simplicial sets to simplicial categories. This functor sends the simplicial set of singular chains on a space X to a category C(X) which has the points of X as objects with morphism space between any two points x and y a simplicial set whose geometric realization is homotopy equivalent to the space of paths in X between x and y. Then we describe a description of a cubical structure on the hom sets of this category and give an explanation how taking normalized chains on these relates to Adams’ classical cobar construction.

This is joint work with Mahmoud Zeinalian.

Date: November 21, 2016

Speaker: Prof. Gabriel Drummond-Cole, IBS Center for Geometry & Physics

Tite: Homotopy probability theory on a Riemannian manifold

Abstract: Homotopy probability theory is a homological enrichment of algebraic probability theory, a toy model for the algebra of observables in a quantum field theory. I will introduce the basics of the theory and use it to describe a reformulation of fluid flow equations on a compact Riemannian manifold.

Date: November 14, 2016

Speaker: Prof. Jae-Suk Park, IBS Center for Geometry and Physics & Postech, Korea

Title: Quantization of the Rational Homotopy Theory

Abstract: We explain how to associate a "classical field theory" to a topological space using a Sullivan algebraic model. We discuss a dictionary between rational homotopy theory and this classical field theory and regard a quantization of the latter theory as a quantization of the former. Finally, the possibility of establishing the concept of “fundamental group” of quantum field theory will be discussed.

Date: October 24, 2016

Speaker: Prof. Misha Gromov, IHES

Title: What is space?

Abstract: I shall try to convince the audience by bringing forth examples that we do not have a satisfactory definition for. I will also speak of non-sufficiency of other mathematical notions..e.g. probability.

Date: April 25,2016

Speaker: Jae-Suk Park, Post tech, Korea

Title: Mastering quantum correlation functions in quantum field theory

Abstract: After a brief review of the quantisation scheme of Batalin-Vilkovisky,

I will discuss the outstanding but rarely acknowledged problem of defining quantum

correlation functions of observables in the scheme and suggest a natural resolution.

From an algebraic package called BV-QFT algebra obtained from a successfully BV quantized

classical field theory, I will show that there is a universal algebraic structure governing every (homotopy) quantum correlation function. The resulting algebraic structure and its relations with the given BV-QFT algebra

will shed some new light on the BV quantization scheme itself as well as motivate us to define category of homotopy CQFT algebras. This leads to the proposal that two BV quantized field theories are physically equivalent if and only if the associated BV-QFT algebras are quasi-isomorphic as homotopy CQFT algebras such that the two theories have isomorphic set of homotopy quantum correlation functions.

Date: November 30, 2015

Speaker: Gabriel Drummond-Cole, IBS Center for Geometry & Physics

Title: Chain level string topology operations

Abstract: I will discuss some details of work with Poirier and Rounds to construct string topology operations on the singular chains of a Riemannian manifold that recover known operations at the level of homology.

Date: November 16, 2015

Speaker: Lev Birbrair, Federal University of Ceara, Brazil

Title: Lipschitz regular complex algebraic sets are smooth

Abstract: A classical Theorem of Mumford implies that a topologically regular complex algebraic surface in C^3 with an isolated singular point is smooth. We prove that any Lipschitz regular complex algebraic set is smooth. No special condition on the dimension and no special condition on the singularity to be isolated is needed. A subset of C^n is called topologically regular if it is a topological manifold with respect to the induced topology and it is called Lipschitz regular, if it is a Lipschitz manifold with respect to the outer metric, i.e. the metric, obtained as a restriction of the ambient metric.

Einstein Chair Seminar Special Friday Meeting

Date: Friday Nov 13, 3-5PM

Room: Science Center/RM 4102

Speaker: David Gepner, Purdue University

Title: Homotopy theory from the viewpoint of infinity-topoi and derived algebraic geometry

Abstract: This talk will focus on various aspects of infinity-topoi and derived algebraic geometry, with emphasis on how they provide a unifying language for many areas of recent interest in homotopy theory. We'll pay particular attention to parametrized and equivariant stable homotopy theory and elliptic cohomology and aim to be elementary, at least in the first part of the talk.

Date: November 2, 2015

Speaker: Jeremy Miller, Purdue University

Title: Localization and homological stability

Abstract: Traditionally, homological stability concerns sequences of spaces with maps between them that induce isomorphisms on homology in a range tending to infinity. I will talk about homological stability phenomena in situations where there are no natural maps between the spaces. The prototypical example of this phenomenon is configuration spaces of particles in a closed manifold. In this and other situations, the homological stability patterns depend heavily on what coefficient ring one considers.

Date: October 26, 2015

Speaker: Uriel Frisch, Lagrange Laboratory University and Observatory Cote d'Azur Nice, France

In format of discussion seminar:

A general survey of history of turbulence. Some remarks on Kraichnan's work who was Einstein's assistant before becoming a guru of tuburlence. Kolmogoroff's work, its verification and refutation, some concrete problems like intermittence.

Date: October 19, 2015

Speaker: Uriel Frisch, Lagrange Laboratory University and Observatory Cote d'Azur Nice, France

Title: The Cauchy-Lagrangian method for numerical analysis of Euler flow and its geometrical content

Abstract: A novel semi-Lagrangian method is introduced to solve numerically the Euler equation for ideal incompressible flow in arbitrary space dimension. It exploits the time-analyticity of fluid particle trajectories and requires, in principle, only limited spatial smoothness of the initial data. Efficient generation of high-order time-Taylor coefficients is made possible by simple recurrence relations that follow from the Cauchy invariants formulation of the Euler equations (Zheligovsky & Frisch, J. Fluid Mech. 2014, vol. 749, 404-430). Truncated time-Taylor series of very high order allow the use of time steps vastly exceeding the Courant-Friedrichs-Lewy limit, without compromising the accuracy of the solution.

Tests performed on the two-dimensional Euler equation indicate that the Cauchy--Lagrangian method is more - and occasionally much more - efficient and less prone to instability than Eulerian Runge-Kutta methods and less prone to rapid growth of rounding errors than the high-order Eulerian time-Taylor algorithm.

At the root of such results lies the geometrical content of the Euler equation as a geodesic flow on the (infinite-dimensional) manifold SDiff of measure-preserving diffeomorphisms. Indeed, the Cauchy invariants may be viewed as a consquence of Noether's theorem applied to the continuous relabelling symmetry. As to the stability of high-order time-Taylor expansions, it follows form Ebin and Marsden's (1970) observation that the geometric formulation of the Euler equation does not lose spatial derivatives.

Date: October 5, 2015

Speaker: Ben Ward, Simons Center for Geometry and Physics

Title: Operads of the Baroque Era

Abstract: The purpose of this talk will be to describe how algebraic structures such as Gerstenhaber, BV, hypercommutive, polyLie or gravity algebras and their up-to-homotopy versions arise naturally from thinking about graphs, surfaces, and operads.

In part 1, we will consider a toy model of the main theorems one dimension down. We will see how a well known fact (the obstruction to an associative algebra being commutative is measured by a Lie bracket) can be viewed as a manifestation of Koszul duality.

In part 2, we will move up a dimension, replacing associative with the little disks, the zero sphere with the circle, and `algebra' with `operad', to make contact with the algebraic structures described above.

Date: September 21, 2015

1:45pm-2:45pm: Speaker: A. Patricia Jancsa, Univ of Buenos Aires

Title: About the classification problem of finite dimensional Lie bialgebras

Abstract: We will recall the notion of Lie bialgebra and Belavin-Drinfel'd result on classification of (factorizable) Lie bialgebra structures on complex simple Lie algebras. I will present a general result on trivial extensions, as follows: for a Lie algebra L = g x V, a direct product of a Lie algebra g and an abelian factor V, I will describe all possible Lie bialgebra structures on L in terms of data on g. The set of biderivations (those which are simultaneously derivations and coderivations) play a fundamental role. The above characterization is complete when g is centerless and without g-invariant antisymmetric 2-tensors, in particular for g as in Belavin-Drinfel'd Theorem, but also for other classes of algebras such as Borel subalgebras of semisimple Lie algebras.

As a second part, I will show how similar methods can be developed for 2-step nilpotent Lie algebras, where no general classification result is known, except for the Heisenberg Lie algebra.

~TEA~

3:15pm-4:15pm: Speaker: Marco Farinati, IMAS

Title: About Lie bialgebra axioms, Gerstenhaber algebras, Cacti-algebras, and H-module algebras.

Abstract: From a Lie algebra g, considering its exterior algebra, that is, the (super)commutative free algebra on g, one obtains a Gerstenhaber algebra. I will first show that to endow g with a Lie bialgebra structure is equivalent to endow the exterior algebra on g with a differential structure; under this equivalence, an involutive Lie bialgebra structure corresponds to a d.g. BV-structure.

Considering the spinless Cactus operad as an intermediate between Gerstenhaber and Gerstenhaber up to homotopy, I will characterize Cacti-algebras which are free as associative algebras, namely, those isomorphic to the tensor algebra on a (eventually graded) vector space V. It turns out that, with a grading hypothesis, they are necessarily the cobar construction on a d.g. associative bialgebra H, with V its augmentation ideal. The key hypothesis on the grading is also satisfied by the Hochschild cohomology complex of an (eventually d.g.) associative algebra A. We will show that Cacti-algebra maps between these two objects are in one to one correspondence with H-module-algebra structures on A. The case where the bialgebra H is the universal enveloping algebra of a Lie g correspond to an action by derivations, and this map gives the subalgebra of Hochschils cohomology generated by these derivations, but, more interesting, there are Hopf algebras without primitive elements (so they don't provide any derivation on A) having nontrivial 2-cocycles.

Date: May 18, 2015

Speaker: Hugo Parlier, University of Fribourg

Title: Graphs, curves and arcs on surfaces

Abstract: There are a number of graphs (and simplicial complexes) associated to surfaces used for a variety of purposes. They are often used to study homeomorphisms groups and deformation spaces of hyperbolic metrics on surfaces but are also intrinsically interesting objects, combinatorial in nature and have a geometry of their own. The talk will be about some of these graphs - namely the curve, pants and flip graphs - for finite and infinite type surfaces.

Date: May 11, 2015

Speaker: John Terilla, Queens and GSUC

Title: Homotopy probability with an application to fluid flow

Abstract: Homotopy probability theory is a variant of probability theory in which the linear space of random variables is replaced with a chain complex. I'll discuss how to use homotopy algebra (rather than analysis) to extract meaningful expectations and correlations among random variables. There will be an emphasis on examples. One example arising in Riemannian geometry naturally connects to fluid flow.

Date: May 4, 2015

Speaker: Somnath Basu, Vivekananda University

Title: Homeomorphism invariants of manifolds & string topology

Date: April 27, 2015

Speaker: Prof. Joel David Hamkins, GC

Title: The continuum hypothesis and other set-theoretic ideas for non-set-theorists

Morning talk: 11:00a-12:45p

The background talk will discuss and prove the theorem of Cantor-Bendixson that every closed set of reals is the union of a countable set and a closed set with no isolated points. The lecture will explain how this result led to Cantor's development of the ordinal numbers and how it establishes that closed sets of reals are either countable or have the same size as the set of all real numbers. We'll see that there are closed sets of arbitrary countable depth in the sense of their analysis. Then ordinals will be discussed: the first uncountable ordinal, the long line, and time permitting Suslin's hypothesis concerning the possibility to characterize the real numbers in terms of its order structure. This will bring the discussion to the topic of the afternoon's session, Cantor's question: Is there an uncountable subset of real numbers whose size is strictly smaller than the size of all real numbers? The "Continuum Hypothesis." is the truth of the negative response to this question.

The organizers thought the discussion just before or during lunch might also touch on the role of set theory and logic s in the foundations of mathematics. Granted that set theory leads to a rich and beautiful part of mathematics, one wonders if there might be something to be gained by seriously developing the logic of a categorical foundation for mathematics which underlies the theory of sets in its role as providing a basis for mathematics. The speaker is prepared to discuss the Feferman theory in comparison to Grothendieck's axiom of universes, and other issues relating set theory to category theory.

Seminar: 2:00p to 3:45/4:00p

Abstract: The main talk returns to set theory per se with a discussion of the Continuum Hypothesis CH: firstly, an explanation of the history and logical status of this hypothesis with respect to the other axioms of set theory, and secondly, recounting the connection between CH and Freiling's axiom AX :

"Let A be the set of functions mapping real numbers in the unit interval [0,1] to countable subsets of the same interval. AX states: For every f in A, there exist x and y in {0,1] such that x is not in f(y) and y is not in f(x)."

The discussion then turns to the axiom of determinacy, including a proof that all open sets are determined and a discussion of the determinacy of Borel sets and the rich logical situation of the axiom with regard to large cardinals. The speaker will illustrate the power of the axiom of determinacy by proving that it implies that every set of reals is Lebesgue measurable.

Then follows a brief mention of the themes and goals of cardinal characteristics of the continuum and of the theory of Borel equivalence relations, those that are defined by countably many of the usual operations on sets starting from open sets. If time permits, some fun geometrical decompositions of space will be displayed that proceed from a transfinite recursion using the axiom of choice, mentioning also the open questions concerning whether or not there are such decompositions that can be defined by Borel subsets.

Date: April 20, 2015

Speaker: Mark Andrea de Cataldo, Stony Brook University

Title: Perverse sheaves and some applications

Abstract: I will first give a motivated introduction to perverse sheaves. I will then discuss a couple of geometric applications to representation theory and combinatorics.

Date: April 13, 2015

Speaker: Santiago Lopez de Medrano, UNAM

Title: Intersections of Quadrics, old and new...

Abstract: Consider the variety defined by the zeroes of k quadratic forms in Rn and its intersection Z with the unit sphere. In 1984 I described the topology of the generic Z for two simultaneously diagonalizable quadratic forms, yet the proof left some cases open. And to extend the result to the non-diagonal case and for k > 2), looked like hopelessly difficult problems.

Up to 2001 there was no real progress on the topology of these varieties and the work derived to the study of some complex manifolds derived from them. But in 2002, F. Bosio and L. Meersseman opened many roads for the case k > 2 and formulated many new problems and an interesting conjecture.

In 2007 Sam Gitler showed me a construction called the polyhedral product functor, which turned out to include all the generic varieties Z as special cases. In 2008 we started working together and proved a much generalized Bosio-Meersseman conjecture and other questions, thus describing a wide (but necessarily partial) region of the k > 2 case. Having opened the gates, a tsunami of results

followed: in a few years I solved (with other collaborators) the rest of cases and problems left open in 1984 and some new ones.

After a review of this story, I will talk about recent work on the projectiviza- tions of those varieties, on a family of them with dihedral symmetry and in the singular ones. In the last case another 30 year old problem was solved.

Date: March 30, 2015

Speaker: Joseph Hirsh, MIT

Title: Universal operad for Riemannian manifolds

Abstract: To any (essentially small) functor to cochain complexes, one can associate a universal operad so that operad maps from O to this universal operad are in 1-to-1 correspondence with functors that lift the original functor to the category of O-algebras (along the forgetful functor). In this talk I'll describe this construction and consider the examples where the diagram category is the category of Smooth or Riemannian manifolds, and the functor is the cochain complex of differential forms.

Date: March 23, 2015

Speaker: Kei Irie, Kyoto University

Title: A chain level BV structure in string topology via de Rham chains

Abstract: We propose a chain model of the free loop space of a

(differentiable) manifold, on which we realize a chain level BV structure in string topology. Namely, it admits an action of a chain model of the framed little disks operad, recovering the Chas-Sullivan BV structure on homology level.

We also discuss relations to Deligne's conjecture for Hochschild complexes. Our construction is based on an idea of de Rham chains, which is a hybrid of singular chains and differential forms.

Date: February 23, 2015

Speaker: Thomas Nikolaus, University of Bonn

Title: Differential cohomology and twisted differential cohomology

Abstract: We give a very general perspective about differential cohomology in terms of non-homotopy invariant cohomology theories. From this perspective we can recover a lot of the structure classically available in differential cohomology (such as the hexagon and the homotopy formula). We will illustrate that perspective with a lot of examples. If time permits we will then talk about twisted differential cohomology and indicate how that can be understood in such a framework. The key of this part will be the classification of differential twists, which we want to describe in some detail.

Date: December 15, 2014

Speaker: Alistair Hamilton, Texas Tech University

Title: Cyclic C-infinity structures and cohomology theories for homotopy algebras based on noncommutative geometry

Abstract:

The so-called cyclic infinity structures were introduced to be the appropriate homotopy invariant generalizations for certain algebraic structures equipped with the extra data of a compatible inner product, playing much the same role as the usual infinity structures do when the bilinear form is not present. In this talk I will prove a remarkable theorem that occurs specifically in the commutative case, that for C-infinity structures there is essentially no difference between these two notions, that is to say that under some reasonably mild conditions we can pass freely from a C-infinity structure to a cyclic C-infinity structure.

The proof utilizes some fairly typical arguments in obstruction theory for which we will employ the cohomology theories that correspond to these types of algebraic structure, as well as making use of the Hodge decomposition of Hochschild cohomology. Here we will advocate a framework for these infinity structures and cohomology theories that is based on the framework of noncommutative symplectic geometry introduced by Kontsevich.

If time permits, we will describe some applications of this theorem to the string topology operations introduced by Chas-Sullivan.

Date: November 17, 2014

Speaker: Yiannis Vlassopoulos (IHES)

Title: Pre Calabi Yau algebras and string topology for manifolds with boundary

Abstract: A Calabi-Yau CY algebra is an A-infinity algebra with a certain kind of duality. An example is the de-Rham algebra of forms on a compact, closed, oriented manifold. A pre-CY (or V-infinity) algebra is a generalization. We shall explain these notions and how the Hochschild chain compex of a pre-CY algebra has the structure of an algebra over a dg-PROP $P$ of chains in the moduli space of Riemann surfaces with incoming and outgoing marked points. We will then show that the de-Rham algebra of forms on a manifold with boundary has the structure of a pre-CY algebra. If the manifold is simply connected this implies that the cohomology of its free loop space has the structure of an algebra over the dg-PROP P and this can be thought of as a version of string topology for manifolds with boundary. This is joint work with Maxim Kontsevich.

Date: November 10, 2014

Speaker: Enrique Pujals, IMPA

Title: Generic dynamics: A phenomena/mechanism correspondence

Abstract: Through geometrical models, pertubation techniques and sometimes analytical approaches, it has been possible to propose and to establish a dictionary between "a taxonomy of generic dynamical phenomenas" and a list of "simple mechanisms or dynamical configurations" responsible for such phenomenas.

In an introductory way, I will try to explain the meaning of such an approach, what is proved in the low differentiability category and what are the (technical?) obstructions to extend those result in high differentiability. During next seminar on Wednesday, I will address some results in the direction to bypass such difficulties.

Date: October 20, 2014

Speaker: Maria Hempel, ETH Zurich

Title: Realization spaces of simply connected polyhedra

Abstract: In this takl I consider the geometric realizations of a polyhedron with a given combinatoric. For simply connected polyhedra I give an explicit parametrization of possible geometric realizations with a mild restriction on the relative position of its edges. If time permits, I will sketch how this description might be used to find polyhedra.

Date: February 3, 2014

Speaker: Alistair Hamilton, Texas Tech University

Title: Compactifications and classes in the moduli space of curves

Abstract: In this talk I will describe a method of producing classes in compactifications of moduli spaces of curves using a framework that appears in quantum field theory under the heading of the Batalin-Vilkovisky formalism. These ideas go back to Kontsevich and his work on noncommutative geometry and Witten's conjecture. Part of the talk will be aimed at explaining some of the relevant background material, such as the orbi-cell decomposition of moduli space and results from the Batalin-Vilkovisky formalism. One of the goals of the talk will be to describe an analogue, dealing with compactifications of the moduli space, of a result due to Kontsevich describing the cohomology of the moduli space in terms of noncommutative geometry. The types of structures producing classes in such compactifications will be related to A-infinity structures, and the role of deformation theory in producing such structures will be explained.

Time permitting, a second construction of classes will be described. The result of pairing the homology and cohomology classes produced by these constructions may be expressed as a functional integral over a finite-dimensional space of fields. Computing such functional integrals can detect the nontriviality of these classes and some examples of this will be described.

Date: October 28, 2013

Speaker: Prof. Ana Rechtman, Université de Strasbourg

Title: A non-amenable Folner foliation

Abstract: In this talk we will discuss the relation between amenable foliations and foliations with Folner leaves. Both notions are motivated by the corresponding ones on finitely generated groups, and in this context they are equivalent. In contrast, for foliations there are examples of non-amenable foliations with all its leaves Folner.

We will begin by discussing the definitions and previously know results.

We will construct an example of a non-amenable foliation with Folner leaves, using a foliated plug motivated by the work of Wilson on the construction of non-singular vector field on 3-manifolds with a finite a set of periodic orbits.

Date: October 21, 2013

Speaker: Daniel Pomerleano, Univ of Tokyo

Title: A user friendly introduction to mirror symmetry

Abstract: From a purely mathematical point of view, mirror symmetry begins with an observation: there is a mysterious symmetry of [p,q] Betti numbers on pairs of six dimensional, compact, algebraic varieties with trivial canonical bundle. This coincidence extends into a dictionary relating holomorphic objects like holomorphic subvarieties and sheaves to symplectic geometry constructions of chain complexes associated to pairs of special sub manifolds of one half the total dimension. The generators are points of intersection between the two submanifolds and the differential is constructed from so called J-holomorphic Riemann surfaces whose boundary lies on these submanifolds and define paths between these points of intersection.

All of these observations demand a geometric explanation. The conjecture of Strominger, Yau and Zaslow is one of the most promising approaches to understanding mirror symmetry and the construction of mirror pairs. From this point of view, mirror pairs involves 2n-manifolds where open dense sets are fibred by n-dimensional torii. These orbits usually develop singularities in the closure of the open dense set--- a typical example is provided by the momentum map of a symplectic manifold equipped with a Hamiltonian torus action of maximal dimension. The conjecture postulates, roughly speaking, that mirror symmetry can be implemented by dualization of torus fibrations. I will give a gentle introduction to this conjecture, focusing on the relevant geometry and topology. Then, I will describe an example, based on joint work with Kazushi Ueda and Kwokwai Chan.

Date: October 7 & 14, 2013

Speaker: Joana Cirici, Freie Universitat Berlin

Title: Cofibrant models of diagrams: applications to rational homotopy

Abstract: Bousfield and Guggenheim reformulated the homotopy theory of differential graded algebras in terms of Quillen model structures, obtaining a description of the homotopy category in terms of homotopy classes of morphisms between minimal objects. Following this line, it would be desirable to establish an analogous formulation for the category of mixed Hodge diagrams: this category appears naturally in the study of the rational homotopy of complex algebraic varieties and involves a mixture of differential graded algebras with (several) filtrations, defined over the fields of rational and complex numbers.

The axioms for Quillen’s model categories are very powerful and they provide, not only a precise description of the maps in the homotopy category, but also higher homotopical structures. As a counterpart, there exist interesting categories from the homotopical point of view, which do not satisfy all the axioms. This is the case of diagram categories involving filtrations, where more specific techniques have to be introduced.

In this talk, I will describe a weaker axiomatic than the one provided by Quillen's model structures, but sufficient to study the homotopy category of certain diagram categories in terms of level-wise cofibrant (or minimal) objects, and to extend the classical theory of derived additive functors, to non-additive settings. The main example of application is the category of mixed Hodge diagrams, but the theory may be applied to other contexts, such as diagrams of topological spaces, or diagrams involving complexes over abelian categories.

Date: September 30, 2013

Speaker: Ralph Kaufmann, Purdue University

Title: Feynman categories: Universal operations and Hopf algebras.

Abstract: After giving a brief definition of Feynman categories. We will discuss how the approach of Feynman categories to operadic type objects naturally leads to universal operations via colimits. Examples are pre--Lie and admissible Lie structures, Gerstenhaber brackets, etc..

We will then present how a co-operad with a mutliplication gives rise to a Hopf algebra structure. This construction unifies Connes-Kreimer Hopf algebras for renormalization, the one of Goncharov for multizetas and the one of Baues in his double co-bar construction.

It is also naturally embedded into the even larger context of Hopf algebras resulting from Feynman categories.

The first part is joint work with B. Ward and the second with I.Galvez-Carrillo and A.Tonks.

Date: May 6, 2013

Speaker: Ana Rechtman, IRMA, Universite de Strasbourg

Title: "The minimal set of Kuperberg's plug"

Abstract: In 1993 K. Kuperberg constructed examples of smooth and real analytic

flows without periodic orbits on any closed 3-manifold. These examples continue to be the only known examples with such properties and are constructed using plugs.

After reviewing K. Kuperberg’s construction, I will present part of a study of the minimal set of Kuperberg’s plug.

Title: "Existence of periodic orbits of geodesible vector fields on 3-manifolds"

Abstract: TBAA non-singular vector eld on a closed manifold is geodesible if there is a Riemannian metric making its orbits geodesics. We will study the existence of periodic orbits for such vector elds on closed 3-manifolds.

On 3-manifolds, K. Kuperberg constructed examples of vector elds without periodic orbits. On the other hand, C. H. Taubes proved that the Reeb vector eld of a contact form has periodic orbits. Reeb vector elds are geodesible, and also suspensions are geodesible. If we assume that the ambient manifold is either dieomorphic to the three sphere or has non trivial second homotopy group, we will prove the existence of a periodic orbit for volume preserving geodesible vector fields.

Volume preserving geodesible vector elds form a subset of the vector elds satisfying the time-independent Euler equations, we will explain how the above result extends to the second family of vector elds.

Date: March 4, 2013

Speaker: Joana Cirici, Freie Universitat Berlin

Title: E1-Formality of Complex Algebraic Varieties

Abstract: I will first recall the construction of the filtration on the cohomology of complex algebraic varieties, called the weight filtration and defined by P. Deligne. I will then introduce the notion of rational homotopy type and, extending the Formality Theorem for the rational homotopy type of compact Kähler manifolds, I will show that every complex algebraic variety (possibly open and/or singular) is E1-formal, which means that its rational homotopy type is entirely determined by the first term of the spectral sequence associated with the multiplicative weight filtration.

Date: February 25, 2013

Speaker: Joana Cirici, Freie Universitat Berlin

Speaker: Dr. Alastair Hamilton Title: Compactifications and classes in the moduli space of curves. Abstract: In this talk I will describe a method of producing classes in compactifications of moduli spaces of curves using a framework that appears in quantum field theory under the heading of the Batalin-Vilkovisky formalism. These ideas go back to Kontsevich and his work on noncommutative geometry and Witten's conjecture. Part of the talk will be aimed at explaining some of the relevant background material, such as the orbi-cell decomposition of moduli space and results from the Batalin-Vilkovisky formalism. One of the goals of the talk will be to describe an analogue, dealing with compactifications of the moduli space, of a result due to Kontsevich describing the cohomology of the moduli space in terms of noncommutative geometry. The types of structures producing classes in such compactifications will be related to A-infinity structures, and the role of deformation theory in producing such structures will be explained. Time permitting, a second construction of classes will be described. The result of pairing the homology and cohomology classes produced by these constructions may be expressed as a functional integral over a finite-dimensional space of fields. Computing such functional integrals can detect the nontriviality of these classes and some examples of this will be described.

Date: February 11, 2013

Speaker: Dan Pomerleano, Kavli IPMU, University of Tokyo

Title: Symplectic homology of affine varieties and string topology

Abstract: We will examine the question of which cotangent bundles have nice symplectic compactifications as projective varieties. We will explain the motivations from deformation theory and mirror symmetry behind conjectures concerning the symplectic homology of symplectic manifolds which have such compactifications. Finally, we will describe what we are able to prove thus far.

Date: December 10, 2012

Speaker: Prof. Bruno Vallette, Universite' NICE

Title: Homotopy theory of algebraic structures

Abstract: The goal of the talk will be to explain how one can do homotopy theory for algebraic structures, i.e. differential graded algebras over an operad, using the Kiszul duality theory for operads. This theory gives cofibrant resolutions (minimal models) for operads and thus define good notions of homotopy algebras with infinity-morphisms. I will use this approach to prove two main theorems: the homotopy transfer theorem and the fact that infinity-quasi-isomorphism are 'invertible'.

Finally, I will endow the category of coalgebras over the Koszul dual cooperad with a model category structure equivalent to the model category of algebras over the original operad. This provides us with a model category structure (without equalizers) on homotopy algebra with infinity-morphisms. In this case, a notion of homotopy relation for infinity-morphisms is given by the Lawrence-Sullivan dg Lie algabra model for the interval.

Date: December 3, 2012

Speaker: Prof. Bruno Vallette, Universite' NICE

Title: Homotopy trivialization of the circle action

Abstract: One can study the homotopy theory of Batalin-Vikovisky algebras using two models of the associated operad: the Koszul model and the minimal model. This allows us to recover and generalize a well-known result of Barannikov-Kontsevich-Manin: the underlying homology groups of a dg BV-algebra satisfying the d/d-bar lemma carry a Frobenius manifold structure. This operadic approach provides us with higher structure which allows us to recover the homotopy type of the original BV-algebra.

We introduced a weaker but optimal condition which ensures the homotopy trivialization of the A-operator (circle action) called "Hodge-to-de Rham degeneration data", which permits us to apply the aforementioned theorem in Poisson geometry and in Lie algebra cohomology. For instance, we can endow the de Rham cohomology of Poisson manifolds (and more generally Jacobi manifolds) with a natural homotopy Frobenius manifold structure (extending the wedge product and allowing one to reconstruct the homotopy type of the de Rham original algebra).

This rich homotopy theory for BV-algebras also gives another interpretation of Givental group action on Cohomological Field Theories (aka Frobenius manifolds) in term of gauge group action. This provides us with a nice relationship between the intersection theory of moduli spaces of curves and the algebraic homotopy theory.

Date: November 26, 2012

Speaker: Nikos Apostolakis, Bronx CC

Title: On (achiral) Lefschetz fibrations over the disk

Series of talks:

Speaker: Tatyana Khodorovskiy, Hunter College

Geometric topology of 4-dimensional manifolds is still much less well understood than the topology of manifolds of other dimensions. It is, after all, the only dimension for which we don't know whether the (smooth) Poincare conjecture is true. Dimension 4 is the highest dimension of so called "low"-dimensional topology. It is not high enough to allow enough room to succumb to powerful theories which work for dimensions 5 and higher, and it is low enough that we can still "picture it" (or at least delude ourselves that we can). Therefore, it can be viewed as a "transition" between high and low dimensional topology. Over the past several decades a good amount of progress has been made in the study of 4-dimensional topology, in these three independent talks, we will discuss the important results, methods and examples in this emerging field.

October 15, 2012: First talk:

In this talk, we will review the history and major results of the topology of 4-dimensional manifolds, as well as discuss the differences with manifolds of dimension other than 4. We will give an outline of the methods used to study 4-dimensional manifolds, and discuss some important invariants that one can associate to a 4-dimensional manifold.

October 22, 2012: Second talk:

Kirby Calculus is a beautiful theory which describes a pictorial method of depicting and manipulating (smooth) 4-dimensional manifolds. This method is very useful for proving positive existence results in smooth 4-dimensional topology. We will draw lots of pictures and give many examples. We will also discuss how these manipulations of 4-dimensional manifolds affect the 3-dimensional manifolds that bound them.

October 29, 2012: Third talk:

Complex surfaces (complex dimension 2 or real dimension 4) are a rich source of examples of 4-dimensional manifolds, and are better understood than general 4-dimensional manifolds. We will give many examples and discuss their classification in relation to the geography of 4-dimensional manifolds.

Date: April 18, 2012

Speaker: Anil Hirani, University of Illinois at Urbana-Champaign

Title: Algebraic Topology for Computations

Abstract: Algebraic topology was once used only within mathematics and physics. Over the last decade or so it has made the transition to high dimensional data analysis, sensor networks, computer graphics, computational geometry, and numerical analysis. I will give two computational examples where algebraic topology brings unification and clarity. But more importantly, in one of the examples it also brings remarkable and unexpected computational efficiency. This has opened up a problem once considered hopelessly intractable by the computational geometry and graphics communities. The other example is computation of harmonic cochains, a fundamental step in vector finite element computations. The first example is the optimal homologous chain problem (OHCP) -- given a chain (usually a cycle), find the smallest one in its homology class. A few years ago it was shown that in mod 2 homology this problem is NP-hard even to approximate. We showed that for integer homology, and posing OHCP as 1-norm minimization, it can be solved in polynomial time for a large class of simplicial complexes. Our main theorem is that the boundary matrix is totally unimodular if and only if the complex is relatively torsion-free. For such complexes linear programming can be used to solve OHCP as an integer program. This is the first appearance of torsion in computational problems. It is also the first topological interpretation of total unimodularity. A geometric interpretation has been known since the early days of linear programming. We've also introduced variants of OHCP and solved an open problem in computational knot theory. We get the second example by replacing 1-norm by 2-norm and homology by cohomology. By using isomorphism between harmonic cochains and cohomology we solve the cohomologous harmonic cochain problem by minimization and get the best known numerical algorithm for it.

Date: February 29, 2012

Speaker: Bhargav Bhatt, Univ of Michigan

Title: Comparison theorems in p-adic Hodge theory

Abstract:A fundamental theorem in classical Hodge theory is the isomorphism between de Rham and Betti cohomology for complex manifolds; this follows directly from the Poincare lemma. The p-adic analogue of this comparison was the subject of a series of conjectures made by Fontaine in the early '80s. In the last three decades, these conjectures have been proven by various mathematicians, and have had an enormous influence on arithmetic algebraic geometry. In my talk, I will first discuss Fontaine's conjectures, and why one might care about them. Then I will talk about some work in progress that leads to a simple conceptual proof of these conjectures based on general principles in derived algebraic geometry, and some classical geometry with curve fibrations.

Date: February 22, 2012

Speaker: Tushar Das, University of North Texas

Title: Infinite-dimensional models of hyperbolic space and related analogues of dynamics and discrete groups

Abstract:We develop the theory of discrete groups acting by hyperbolic isometries on the open unit ball of an infinite-dimensional separable Hilbert space. We generalize most results of negative curvature and Gromov-hyperbolic settings to get to their geometric core and have greater scope for applications. Many of the essential ideas are already present when working in Hilbert space, although one must be careful with boundaries and non-geodesic scenarios. There are many examples that explain what is fundamentally different from the classical finite-dimensional setting. For starters, in infinite dimensions properly discontinuous actions are no longer strongly discrete (finitely many orbit points in arbitrary balls) and though a Poincare-type summation over the orbits being finite implies strong discreteness always, the reverse fails in infinite-dimensions. The existence of fixed points of isometries and their structure will be discussed - here one discovers interesting parabolic behaviour that's absent in finite dimensions. We characterize groups whose limit sets are compact and convex-cobounded in terms of radial points in the limit set. Schottky groups whose limit sets are Cantor sets provide a variety of interesting phenomena where extensions of the classical thermodynamic formalism (a la Bowen) prove strong results about the geometry and dynamical properties of their limit sets. We prove a generalization of the Bishop-Jones theorem, equating the Hausdorff dimension of the radial limit set with the Poincare exponent. Time permitting, we sketch the proof of the Ahlfors-Thurston theorem and develop Patterson-Sullivan theory for divergence type groups. Here there are examples of convergence type groups that do not admit a conformal measure. To end, we discuss a few problems/applications. Almost everything will be developed from scratch with an attempt to present the underlying geometric ideas behind the proofs – graduate students are very welcome.

Date: February 1, 2012

Speaker: Daniel Pormeleano, University of Berkeley

Title: Curved Topology and Tangential Fukaya Categories

Abstract:In this talk, we will look at non-commutative versions of Landau-Ginzburg models. More precisely, given a simply connected manifold M such that it’s cochain algebra, C^*(M), is a pure Sullivan dga, we will consider curved deformations of the algebra of chains on it’s loop space C_* (\Omega(M)) and consider when the category of curved modules over these algebras becomes fully dualizable. For simple manifolds, like products of spheres, we are able to give an explicit criterion, like the Jacobian criterion, for when the resulting category of curved modules is smooth, proper and CY and thus gives rise to a TQFT. We give Floer theoretic interpretations of these theories for projective spaces and their products, which involve defining a Fukaya category which counts holomorphic disks with prescribed tangencies to a divisor.

Date: November 30, 2011

Speaker: Justin Young, Indiana University

Title: Brace Bar-Cobar Duality and the E_2 cochain algebra

Abstract: After providing motivation for studying the E_2 algebra (roughly, d.g. algebra plus homotopy commutative) structure on the cochain complex of a space, we will consider the category of dg E_2 algebras. We will show that the classical bar-cobar duality between dg algebras and dg coalgebras can be enhanced to a duality between dg E_2 algebras and dg Hopf algebras. Then, we will discuss application of this duality to finding a commutative/Lie model for a space.

Date: November 9, 2011,

**Please note time change below**

Speaker: 1:45pm - 2:45pm: Prof. Alessandra Iozzi, ETH Zurich

Speaker: 2:50pm - 3:50pm: Prof. Marc Burger, ETH Zurich

Title: Weakly Maximal and Casual Representations of Surface Groups, I and II

Abstract:We define the notion of a weakly maximal representation of a surface group and relate it to the previously studied notions of a maximal representation and a tight homomorphism. While these are defined in terms of bounded cohomology, we describe a particular kind of weakly maximal representation arising from a purely geometrical construction, namely the causal representations. We then present a structure theorem for weakly maximal representations, which leads to a new characterization of Teichmüller space.

Date: November 2, 2011

Speaker:Prof. Jae Suk Park, Yonsei University

Title: Homotopical Probability Space

Date: October 26, 2011

Speaker: Prof. Maria Hempel, ETH Zurich

Title: Rigidity of Surfaces:The Polyhedral case

Date: October 19, 2011

Speaker: Prof. Robert Guralnick, University of Southern California

Title: Maps from the Generic Riemann Surface

Abstract: In 1936, Zariski proved that for g greater than 6, there is no holomorphic map from the generic Riemann surface of genus g to the Riemann sphere so that the monodromy group of the branched covering representing the map is solvable. It was well known that the generic Riemann surface of genus 6 has a degree 4 map to the sphere and so its monodromy group is solvable. We generalize this by showing that an indecomposable map from the generic Riemann surface of genus g at least 4 of degree n to the sphere (that is one which is not a composition of branched coverings) has monodromy group an alternating or symmetric group of degree n at least 2g or greater than g/2 respectively. We will also discuss the case of rational maps on the Riemann sphere as well as analogs in positive characteristic.

Date: May 18, 2011

Speaker: Prof. Gregory Ginot, Université Pierre et Marie Curie

Title: Beilinson-Drinfeld algebras, Quantization and Todd genus

Abstract: Beilinson-Drinfeld algebras are a structure describing the observables of a quantization of a classical field theory. Following, Costello, Grady-Gwilliam, we will explain how the Todd genus is encoded in the existence of a quasi-isomorphism between two Beilinson-Drinfeld algebras, which, as spaces, are given by Hochchild homology of the Weyl algebra and the de Rham forms. This quasi-isomorphism follows from a general theorem on quantization of observables applied to a real Chern-Simons field theory that we will describe.

Date: May 11, 2011

Speaker: Prof. Gregory Ginot, Université Pierre et Marie Curie

Title: Factorization algebras and invariants of framed manifolds

Abstract: We will recall one definition of topological chiral homology, which is an invariant associated to framed manifolds of dimension n and E_n-algebras introduced by Lurie. We will explain how this invariant can be computed as a factorization algebra homology and can be extended to higher Hochschild homology when the algebra is actually commutative. We will also explain a few elementary consequences of these equivalences. This is joint work with Tradler and Zeinalian.

Date: May 4, 2011

Speaker: Prof. Gregory Ginot, Université Pierre et Marie Curie

Title: Factorization algebras and examples

Abstract: We will introduce the definitions of Factorization Algebras, which are a kind of "multiplicative" cosheaf studied by Costello-Gwilliam. Factorization algebras are closely related to several classical objects of study in algebraic topology such as mapping spaces, E_n-algebras, Hochschild homology and arise as well when studying the observables of (quantum) field theories. We will explain more precisely the aforementioned relationships and will give some elementary properties, as well as examples, of factorization algebras.

Date: March 23, 2011

Speaker: Prof. Ezra Getzler, Northwestern University

Title: A filtration of open/closed topological field theory

Abstract: we prove a higher analogue of the presentation of modular functors due to Moore and Seiberg, and extend it to open/closed field theory.

Date: March 2, 2011

Speaker: Prof. Albert Fathi, École Normale Supérieure de Lyon

Title: Aubry-Mather Theory, Lax-Oleinik semi-group, and viscosity solutions of the Hamilton-Jacobi Equation

Abstract: For Lagrangian systems Aubry-Mather theory is about what remains when KAM theorem cannot be applied. KAM theorem provides invariant tori which are Lagrangian, hence they are smooth solutions of the Hamilton-Jacobi theory. As was discovered 15 years ago, when there are no smooth solutions weak solutions come into play and Aubry-Mather theory can be recovered from them: this is weak KAM theory. The lecture will give an introduction to the subject; in the second part we will provide details, and some more recent developments.

Date: December 8, 2010

Speaker: Prof.Daniel Meyer, Helsinki University

Title: Invariant Peano curves of Expanding Thurston maps

Abstract: Let f be a self mapping of the two sphere which is a branched covering. We assume that the orbits of the critical points are finite and that the map is expanding in the sense that the components of the iterated preimage of a certain simply connected domain have diameters tending to zero. We show a sufficiently high iterate F of f is the quotient of the expanding self mapping g of the circle with the same degree as F. This means there an intertwining mapping C from the circle onto the two sphere (i.e. a Peano curve) so that FC=Cg. This generalizes a result of Milnor and corresponds to a result by Cannon-Thurston in the Kleinian group case. Furthermore F can be obtained by a geometric construction called "mating". This construction due to Douady-Hubbard is a way to geometrically combine two polynomials to form a rational map.

Date: November 17, 2010

Speaker: **CANCELLED** 3pm: Prof. Yakov Sinai, Princeton University

Title: Renormalization group technique and burgers system

Abstract: The talk will be about the construction of solutions of the Burgers systems which develop singularities in finite time. It will consist of five parts: 1. General Outline of RGM 2. Burgers system, a survey. 3. The derivation of the equation for the fixed point. 4. The linearized spectrum. 5. Main result of blow ups in complex-valued solutions of BS.

Speaker: 3:30-4:40pm: Prof. Yevsey Nisnevich, Courant Institute

Title: Adeles, Grothendieck topologies & G-buncles for semisimple groups G

Abstract: Conclusion of mini-course (SIMONS LECTURES/EINSTEIN CHAIR LECTURES/ARITHMETIC GEOMETRY TEAM LECTURES)

Date: November 10, 2010

Speaker: 1:30-3:15pm: Prof. Ralph Kaufmann, Purdue University

Title: Algebraic structures from odd operads and real blow ups

Abstract: Some classical algebraic structures like Gerstenhaber's bracket on the Hochschild complex have an operadic origin. We discuss generalizations of these operations coming from different operadic type settings. This includes BV operators and master equations. The natural setting for these considerations is the theory of (twisted) generalized operads. We also relate these constructions to the topological level by using moduli spaces and real blow-ups. This is joint work with Ben Ward and Javier Zuniga.

Speaker: 3:30-4:40pm: Prof. Yevsey Nisnevich, Courant Institute

Title: Adeles, Grothendieck topologies & G-buncles for semisimple groups G

Abstract: Continuation of mini-course (SIMONS LECTURES/EINSTEIN CHAIR LECTURES/ARITHMETIC GEOMETRY TEAM LECTURES)

Date: November 3, 2010

Speaker: 1:30-3:15pm: Joseph Hirsh, GSUC

Title: Relative Categories, Infinity Categories, and Simplicial Localization

Abstract: We will describe a passage, called simplicial localization, from categories Cwith collections of "weak equivalences" W to (infty,1) categories. We will explain how this passage can be seen as an equivalence between two versions of the theory of homotopy theories. Time permitting, we will introduce another (equivalent) version of the simplicial localization which is more useful for computations.

Speaker: 3:30-4:40pm: Prof. Yevsey Nisnevich, Courant Institute

Title: Adeles, Grothendieck topologies & G-buncles for semisimple groups G

Abstract: Continuation of mini-course (SIMONS LECTURES/EINSTEIN CHAIR LECTURES/ARITHMETIC GEOMETRY TEAM LECTURES)

Date: October 27, 2010

Speaker: 1:30-3:15pm: Prof. Scott Wilson, Queens College

Title: Period extensions of holonomy to mapping spaces.

Abstract: Bismut showed that holonomy, defined on the loopspace of a manifold with bundle and connection, extends to a closed periodic form on the loopspace of the manifold. Moreover, the restriction of this class to constant loops gives the classical chern character of the bundle. I will begin this talk by giving a new presentation of this result using a variation on hochschild complexes and iterated integrals that is suitable for locally trivialized bundles. Next, I will introduce the notion of an abelian gerbe with connection. A similar variation of higher hochschild complexes and their iterated integrals will be applied to show that, for an abelian gerbe with connection over M, the 2-holonomy of the connection extends to a closed periodic form on the space M^T of maps of a torus into M. Finally, it's known (and I'll explain) how an abelian gerbe with connection on M induces a line bundle with connection on LM, and I'll describe how the two constructions from above, on LLM and M^T, are related. This is joint work with Thomas Tradler and Mahmoud Zeinalian.

Speaker:3:30-4:40pm: Prof. Yevsey Nisnevich, Courant Institute

Title:Adeles, Grothendieck topologies & G-buncles for semisimple groups G

Abstract: Continuation of mini-course (Please see below)

SIMONS LECTURES/EINSTEIN CHAIR LECTURES/ARITHMETIC GEOMETRY TEAM LECTURES

Speaker: Prof. Yevsey Nisnevich, Courant Institute

Title: Adeles, Grothendieck topologies & G-bundles for semisimple groups G

Locations: STONYBROOK MATHEMATICS AND CUNY GRADUATE CENTER

{TWO PARTS EACH DAY AND INDEPENDENT AT EACH SITE}

STONYBROOK 2 & 3:30pm [Tues, Oct 12 & Fri, Oct 15: Room 4-125] [Mon, Oct 18 & Friday Oct 22: Room P-131]

CUNY GRAD CENTER [ROOM 6417] WED 3:30 & 5pm October 13, 20

OPEN TO ALL- EXPERTS AND NON EXPERTS

1) Preliminaries on schemes: Noetherian schemes, closed, non-closed and generic points, the local rings and the residue fields of points, the structure sheaf. The Krull dimension of schemes. The functor of points Y -> X(Y) of a scheme X with the values in another (variable) scheme Y and the Grothendieck-Yoneda embedding. Fiber product of schemes, fibers of a morphism of schemes f: X -> Y.

2) Some important classes of morphisms of schemes: unramified, flat, etale and smooth morphisms. Regular and singular points on a scheme.

3) Grothendieck topologies: the general definition and some functorial properties. The principle examples: the Zariski, Nisnevich, etale and faithfully flat quasi-compact topologies on the category of schemes and its subcategories (in the increasing order). The concepts of points and local rings of points for Grothendieck topologies. Sheaves on a Grothendieck topology and a fiber of a sheaf over a point on a topology. Henselian rings and their main properties, Henselization of a ring. The description of local rings and fibers of sheaves over points for the first 3 topologies listed above and appearance of Henselian rings for the Nisnevich and etale topologies.

4) Cohomology of sheaves of abelian groups on a Grothendieck topology. Some functorial properties. Cohomological dimensions of the principle topologies.

5) The principal homogeneous spaces for a sheaf of (possibly) non-abelian groups G on a topology \tau, locally trivial in this topology (= \tau G-torsors). Cohomology of non-abelian sheaves on a topology \tau and non-abelian exact sequences for a sheaf of subgroups H of G.

6) Group schemes - definition and the principle examples: additive G_a, multiplicative G_m, algebraic tori, full linear group GL_m over an arbitrary base scheme. Semisimple and reductive group schemes over a base scheme.

7) Adeles for schemes of dimension 1 with values in a group scheme G. Henselian adeles. The Nisnevich topology as a mean for a geometrization of adeles. Nisnevich cohomology and adelic class groups.

8) The principle vanishing theorems for the class groups and the Nisnevich and etale cohomology of semisimple group schemes over general Dedekind (= regular 1-dimensional) rings. Cohomological expressions for these invariants in the non-simply connected case. Finiteness theorems for Dedekind rings with finitely generated divisor class groups.

9) Applications of the vanishing theorems: proof of the conjecture of Grothendieck and Serre on the Zariski local triviality of rationally trivial etale G-torsors over regular base of dimension 1 and 2 and for Henselian local regular rings of an arbitrary dimension.

10) Further applications: extensions of the Uniformization Theorem for the moduli space of G-vector bundles over a smooth irreducible projective curve C/k onto arbitrary fields k and general semisimple group schemes G over C. (It was previously known for an algebraically closed field k and a constant semisimple group scheme G/k only and it was due mainly to Drinfeld-Simpson (1995).

Date: September 22, 2010

Speaker: 2-2:50pm-Prof. Edson de Faria, University of Sao Paulo

Title: Unbounded conformal distortion - David homeomorphisms via Carlson boxes

Abstract: We construct a family of examples of increasing homeomorphisms of the real line whose local quasi-symmetric distortion blows up almost everywhere, which nevertheless can be realized as the boundary values of David homeomorphisms of the upper half-plane. The construction of such David extensions uses Carlson boxes.

Speaker: 3-3:50pm-Prof. Jae-Suk Park, Research Institution, Seoul

Title: Algebraic Models of QFT and corresponding Homotopy Invariants

Abstract: This talk is about an attempt to understand quantum field theory mathematically. I shall explain the notion of a binary QFT, which shall be viewed as an algebraic model for quantum field theory with a binary product. First a binary QFT algebra is a certain algebraic structure defined over k[[h]] such that its classical limit (h=0) is a commutative differential graded algebra (CDGA) over k. Then a binary QFT is a binary QFT algebra with a QFT cycle, whose classical limit is a cycle of the CDGA. I shall define notions of quantized observables and their quantum expectation values and quantum correlation functions as certain homotopy invariants of this algebraic setup. An exact solution for all quantum correlation functions shall be presented under certain conditions. Such a theory is part of a natural family parametrized by a smooth-formal moduli space which has certain coordinates. These generalize those of flat or special coordinates in topological string theories. Time permitting I shall briefly explain a program to go from or quantize a CDGA with "cycle" to obtain a binary QFT.

Date: September 1, 2010

Speaker: Prof. Qian Yin, University of Michigan

Title: Expanding Thurston Maps and Cell Decompositions

Abstract: Thurston maps are branched covering maps over the 2-sphere with finite post-critical sets. In this talk, we are going to define expanding Thurston maps and give natural cell-decompositions of the 2-sphere induced by the dynamics of an expanding Thurston map following Bonk and Meyer. These cell decompositions help us understand the structure of expanding Thurston maps. As time permits, we will see how these cell decompositions give us a Gromov hyperbolic space, and describe some very nice properties of it.

Date: May 19, 2010

Speaker: Prof. John Baez, University of California Riverside

Title: Electrical Circuits

Abstract: While category theory has many sophisticated applications to theoretical physics — especially quantum fields and strings — it also has interesting applications to a seemingly more pedestrian topic: electrical circuits. The pictorial resemblance between circuit diagrams and Feynman diagrams is an obvious clue, but what is the underlying mathematics? This question quickly leads us to an interesting combination of category theory, symplectic geometry, complex analysis and graph theory. Moreover, electrical circuits are just one example of 'open systems': physical systems that interact with their environment. While textbooks on classical mechanics usually focus on closed systems, open systems are more important in engineering, and their mathematics is arguably deeper and more interesting.

Date: April 28, 2010

Speaker: Prof. H.T. Yau, Harvard University

Title: Lattice gases, large deviations, and the incompressible Navier-Stokes equations

Abstract: We study the incompressible limit for a class of random particle systems on the cubic lattice in three space. For starting probability distributions corresponding to arbitrary macroscopic finite energy initial data the distributions of the evolving empirical momentum densities are shown to have a weak limit supported entirely on global weak solutions of the incompressible Navier-Stokes equations. Furthermore explicit exponential rates for the convergence (large deviations) are obtained.

Date: April 21, 2010

Speaker: Prof. Gregory Ginot, Université Pierre et Marie Curie

Title: "Hochschild chain complex over spaces and topological chiral homology."

Abstract: This is a joint work (in progress) with T. Tradler and M. Zeinalian. We will review the notion of Hochschild chain complex over spaces, following Pirashivili, which, roughly speaking, is a (homotopy) bifunctor from the category of commutative algebras and the category of spaces to the category of DG-commutative algebras. We will explain how this theory can be interpreted as a special kind of (unoriented) extended (\infty,n)-dimensional topological field theories and how it is related to topological chiral homology in the sense of Lurie.

Date: March 10, 2010

Speaker: David Chateur, Universite' de Lille

Title: "On Bivariant chains of PL-manifolds"

Abstract: Let M be a closed oriented PL-manifold, in this talk we will present a bivariant chain theory for M. This bivariant chain complex is naturally quasi-isomorphic to the PL-chains of M and its singular cochain complex via a nice chain model of the Poincaré duality map. We will apply this construction to show that Mc-Clure's partial commutative intersection product is equivalent to the cup product.

Date: February 24, 2010

Speaker: Alexander Shnirelman, Concordia University

Title: "Long-time behavior of 2-dimensional flows of ideal incompressible fluid"

Abstract: Consider the motion of ideal incompressible fluid in a bounded 2-d domain. It is described by the Euler equations which, in spite of their deceptive simplicity, are hard to investigate. For the initial velocity field smooth enough, the Euler equations have a unique solution for all time, and it's natural to ask what is its long-time asymptotics. The physical experiments and computer simulations show a nontrivial, counterintuitive picture of a huge attractor in the space of incompressible velocity fields, consisting of stationary, periodic, quasiperiodic and, possibly, chaotic solutions. This picture appears to contradict the conservative nature of the Euler equations; this is similar to contradiction between the microscopical reversibility of the molecular motion and macroscopical irreversibility of thermodynamical processes. I am going to demonstrate the results of computer simulation and physical experiments on the fluid motion, and discuss connections of this problem with analysis, dynamical systems and even topology.

Date: February 10, 2010

Speaker: Ruth Lawrence, Hebrew University

Title: "On quantum knot and 3-manifold invariants"

Abstract: Given a semi-simple Lie algebra and a choice of representation on each component, there is defined an invariant of links in the 3-sphere as a polynomial in a parameter q known as the colored Jones polynomial. Using a surgery presentation of 3-manifolds, this can be extended to invariants of compact oriented 3-manifolds (and more generally of links embedded in such 3-manifolds) dependent on a root of unity q, namely the Witten-Reshetikhin-Turaev (WRT) invariant. The Ohtsuki invariant of rational homology spheres is a formal power series-valued invariant which may be constructed out of congruence properties of WRT invariants, and should be considered as an asymptotic expansion of the WRT invariant, in a suitable sense. Collectively these invariants are known as quantum invariants, as distinct from classical topological invariants obtainable with classical techniques of homology and homotopy. This will be a survey talk in which we define and discuss properties and structure of these invariants, ranging across integrality, congruence, almost modularity, holomorphicity and asymptotic structure. No prior knowledge of quantum invariants will be assumed.

Date: February 3, 2010

Speaker: Borya Shoiket, University of Luxembourg

Title: "What is the categorical generalization to bialgebras of the monoidal category of bimodules over an algebra."

Abstract: Let B be an associative algebra with a coassociative coalgebra structure which is compatible in the sense that the comultiplication is a map of algebras. In other words B is an associative and coassociative bialgebra,or bialgebra for short. Such an algebraic structure has a deformation theory which is controlled by a "deformation complex" D(B), in particular its second cohomology. D(B) is called the Gerstenhaber-Schack complex for its creators Gerstenhaber and Schack at the University of Pennsylvania. It is conjectured that D(B) has a homotopy commutative product and a homotopy Lie bracket of degree -2 and that these are compatible by a homotopy derivation property. So far, no explicit construction of any of these pieces of the structure is known. We will present a construction of a structure on the cohomology of D(B) of the sort that would result if the conjecture were true. To do this we make the additional assumption that B is a hopf algebra. This means there is an anti-automorphism S intertwining multiplication and comultiplication in a diagram you can see on wikipedia. An analogous contruction in the case of the complex controlling deformations of an associative algebra A (the Gerstenhaber- Hochshild complex) is due to S. Schwede and uses the monoidal structure on the category of A-bimodules. In particular, Schwede gives a conceptual construction of the Gerstenhaber bracket on the corresponding Gerstenhaber-Hochschild cohomology. In the case of bialgebras what replaces the category of bimodules is the category of tetramodules. This category admits two different monoidal structures. These two structures are compatible in a rather non-trivial way. Tetramodules over B have a 2-monoidal category structure. We will prove the following general theorem: let Q be an n-monoidal abelian category (with some mild assumptions), and let e be the unit object in Q. Then Ext/Q (e,e) has the structure that would result if the complex defining the Ext had a commutative product and compatible lie bracket of degree -n and all this up to homotopy.

Date: November 18, 2009

Speaker: Zhenghan Wang, Senior Researcher, Microsoft Corporation

Title: "Pictures of curve relations on surfaces can define 2+1 topological quantum field theories"

Abstract: Curves in surfaces are deceptively elementary and simple objects. By considering linear combinations of simple closed curves in surfaces and relations among them, we arrive at beautiful (2+1)-TQFTs, the so-called diagram TQFTs. Diagram TQFTs with specific idempotents due to Jones and Wenzlas relations can be considered as quantum generalizations of linearized mod two- homology. Using picture relations among trivalent graphs instead of simple closed curves, we can construct all of the (2+1)-TQFTs related to the constructions of Drinfeld. Such TQFTs are not only interesting in mathematics, but they are also likely to play a role in condensed matter physics, even possibly in the construction of quantum computers. If time permits, I will discuss possible applications to quantum computing.

Date: October 21, 2009

Speaker: Joan Millès, Université Nice

Title: "André-Quillen cohomology theory of an algebra over an operad"

Abstract: Following the ideas of Quillen and by means of model category structures, Hinich, Goerss and Hopkins have developped a cohomology theory for (simplicial) algebras over a (simplicial) operad. Thank to Koszul duality theory of operads, we describe the cotangent complex to make these theories explicit in the differential graded setting. We recover the known theories as Hochschild cohomology theory for associative algebras and Chevalley-Eilenberg cohomology theory for Lie algebras and we define the new case of homotopy algebras. We study the general properties of such cohomology theories and we give an effective criterium to determine whether a cohomology theory is an Ext-functor. We show that it is always the case for homotopy algebras.

Date: April 22, 2009

Speaker: Gregory Ginot, Université Pierre et Marie Curie

Title: "Hochschild (co)homology over spaces and String Topology"

Abstract: We will explain how one can define Hochschild (co)chain complex associated in a functorial way to any space X, CDG algebra A and A-module M. We will explain the relationships between these Hochschild (co)homology theories and string (or Brane or Surface) topology.

Date: March 11, 2009

Speaker: Max Lipyaskiy, Columbia University

Title: "Geometric cycles in classical topology and Floer theory"

Abstract: I will introduce a new approach to Floer theory based on mappings of Hilbert manifolds into a target space. After describing the general framework for the theory, I will discuss the relationship of the Floer theory of the cotangent bundle of a manifold to the classical homology its loopspace.

Date: December 3, 2008

Speaker: Victor Turchin, Kansas State University

Title: "Hodge decomposition in the homology of long knots" (Joint work in progress with G. Arone and P. Lambrechts)

Abstract: We will describe a natural splitting in the rational homology and homotopy of the spaces of long knots Emb(R^1,R^N). This decomposition arises from the cabling maps in the same way as a natural decomposition in the homology of loop spaces arises from power maps. The generating function for the Euler characteristics of the terms of this splitting will be presented. Based on this generating function one can show that both the homology and homotopy ranks of the spaces in question grow at least exponentially. There are two more motivations to study this decomposition. First, it is related to the study of the homology of higher dimensional knots Emb(R^k,R^N). Second, it is deeply related to the question whether Vassiliev invariants can distinguish knots from their inverses.

Date: November 19, 2008

Speaker: Prof. Carl-Friedrich Boedigheimer, Mathematical Institute University of Bonn

Title: "Models for the Moduli Spaces of String Theory"

Abstract: Let M = M(g,m,n) be the moduli space of surfaces of genus g with n incoming and m outgoing boundary curves. Theses moduli spaces have attracted much attention in recent years for their importance in string theory (either of physical or mathematical origin). We shall give a description of M as a finite cell complex. The cells are given by simultaneous conjugation classes of q-tuples of permutations in the p-th symmetric group, where p < 2h + 1, q < h + 1, and h = 2g - 2 + n + m, and the number of cycles of the permutation in the last component of the q-tuple (respectively, the permutation in the first component) is n (respectively, m). Furthermore, we shall describe the operad structure of all such moduli spaces in terms of these models.

Date: November 12, 2008

Speaker: Prof. Jean Louis Loday, CNRS et Université de Strasbourg

Title: First talk: "Combinatorial Hopf algebras".

Abstract: Many recent papers are devoted to some infinite dimensional Hopf algebras called collectively "combinatorial Hopf algebras". Among the examples we find the Faa di Bruno algebra, the Connes-Kreimer algebra and the Malvenuto-Reutenauer algebra. We give a precise definition of such an object and we provide a classification. We show that the notion of preLie algebra and of brace algebra play a key role.

Second talk: "Homotopy associative algebras and Stasheff polytope".

Abstract: We construct an A-infinity algebra structure on the tensor product of two A-infinity algebras by providing a simple formula for a geometric diagonal of the Stasheff polytope. This formula is based on the Tamari poset structure on the set of planar binary trees. As a result the operad A-infinity gets a binary cooperation. We show that similar formulas give higher cooperations so that the operad A-infinity gets a structure of A-infinity coalgebra for the Hadamard product.

Date: October 22, 2008

Speaker: Prof. Alastair Hamilton, Univ. of Connecticut

Title: "Noncommutative geometry, compactifications of the moduli space of curves and A-infinity algebras."

Abstract: There is a theorem, due to Kontsevich, which states that the homology of the moduli space of curves can be identified with the homology of a certain infinite dimensional Lie algebra. This Lie algebra is constructed as the noncommutative analogue of the Poisson algebra of hamiltonian vector fields on an affine symplectic manifold. There is a compactification of the moduli space of curves which was introduced by Kontsevich in his study and proof of Witten's conjectures. It is defined as a certain quotient of the well-known Deligne-Mumford compactification. In the first part of this talk I will describe how Kontsevich's Lie algebra can be deformed into a differential graded Lie algebra whose homology recovers precisely the homology of this compactification of the moduli space. This is achieved through the use of an additional structure on this Lie algebra -- a Lie cobracket -- which makes Kontsevich's Lie algebra into a Lie bialgebra. Such structures have been considered by various authors including Chas-Sullivan, Movshev, Fukaya and Ginzburg-Schedler. I will explain how the relationship between the moduli space and its compactification is reflected algebraically in this framework -- the deformation parameters contain the extra information at the boundary of the moduli space. In the second part of the talk I will explain how the definition of an A-infinity algebra and one of its important generalisations known as a cyclic A-infinity algebra can be subsumed in this framework of noncommutative geometry using the notion of Maurer-Cartan moduli space. I will explain a simple construction which produces classes in the homology of any differential graded Lie algebra by exponentiating elements in its associated Maurer-Cartan moduli space. This construction can be used to produce classes in the moduli space from cyclic A-infinity algebras, as was observed by Kontsevich. The corresponding algebraic structures producing classes in the compactification of this moduli space seem to sometimes go under the heading of `quantum A-infinity algebras'. There is a natural deformation theory which controls the process of building a quantum A-infinity algebra out of a cyclic A-infinty algebra. I will explain how the problem of building a quantum A-infinity algebra out of a cyclic A-infinity algebra corresponds to the problem of extending a class defined on the moduli space to its compactification. I will explain how these ideas apply to a simple but important example.

Date: September 17, 2008

Speaker: Prof. Vasiliy Dolgushev, UC Riverside

Title: "Proof of Swiss cheese conjecture"

Dates: September 10 - September 14, 2008

Speakers: Please see the schedule

Title: "FRG CUNY Workshop"

To view (please allow time to download talks): http://vvf.streamhoster.com/ViewVirtualFolder.aspx?vfid=5e8cab41-9c97-4f03-af50-3c1d117b86d9

Date: May 28, 2008

Speaker: Nathan Habegger, Université de Nantes

Title: "Vassiliev invariants and related invariants in 3 dimensions"

Date: May 21, 2008

Speaker: Michael Freedman, Microsoft

Title: "Positivity of the universal pairing in dimension=3 "

Abstract: This will be a mathematics talk explaining arxive:math/0802.3208. The topic is a positivity property of the sesquilinear pairing defined by gluing a superposition of three manifolds with a fixed boundary Y to a superposition with boundary (Y bar) - Y with reversed orientation. Proof uses all aspects of the theory of 3-manifolds from Dehn to Thurston to Witten to Perelman. One motivation for this study comes from condensed matter physics. I mention it bellow in the hope that it may attract a few physicists to the lecture (who would also be welcome to leave at any point they choose as the discussion will necessarily be in the language of topology.) Surface layer physics, particularly the two-dimensional electron gasses which generate the fractional quantum Hall effect (FQHE), are presently being investigated as a possible substrate for the construction of a quantum computer. The mathematical concept of a (2+1)-dimensional Unitary topological quantum field theory (UTQFT) provides the link between the lowest energy properties of surface layer physics and topology. Under this mapping developed by Witten, Read, Moore, and others, a bounded 3-manifolds map to quantum mechanical state on the bounding surface. A consequence of a celebrated result of Cumrum Vafa (Harvard) that for no single UTQFT is this mapping "injective," i.e., separates all three manifolds with fixed boundary. A crucial question is whether, taken together, a family of UTQFTs might successfully reflect all of 3-manifold topology (that is, be injective). This paper (arxiv:math/0802.3208) by Freedman, Walker, and Calegari shows that this is, at least, possible by producing a complexity function "c" on closed 3-manifolds with the same formal property for gluings: c(AB) or= max( c(AA), c(BB)) with equality holding only if A=B, as would a partition function of a UTQFT which had been (miraculously) liberated from the constraints of Vafa's theorem. This situation is in direct opposition to the state of affairs in 3+1 dimensions. There (arxiv:math/0503054) it was found that much of the interesting detail of 4-manifolds (Donaldson and Seiberg-Witten invariants) were not reflected in the structure of (3+1)-dimensional UTQFTs. This establishes a fundamental distinction between the quantum mechanics of two- and three-dimensional systems.

Date: May 14, 2008

Speaker: Tom Lada, North Carolina State University

Title: "Homotopy Algebras and Brace Algebras "

Abstract: We will review the concept of L-infinity algebras from several points of view, including the relationship with brace algebras. We will also discuss several types of actions of such algebras, such as L-infinity modules and OCHAS (open closed homotopy algebras). Several concrete examples will be exhibited.

Date: April 23, 2008

Speaker: Jae-Suk Park, Yonsei University, Seoul, Korea

Title: "Minimal model of QFT "

Abstract: This talk is about an effort to understand quantum field theory (QFT) mathematically by studying what we call commutative quantum algebras (CQA). We begin with a simple but important example of CQA; Fix a ground field k with char(k)=0, and let h be a formal parameter. Def) A BV quantum algebra is a triple (C[[h]], K, m), where 1. (C[[h]], m) is a graded commutative associative k[[h]]-algebra, where C[[h]] is free as a k[[h]]-module such that (C= C[[h]]/h C[[h]], m ) is a graded commutative associative k-algebra, 2. (C[[h]],K) is a cochain complex over k[[h]]. 3. The failure of K being a derivation of m is a derivation of m and is divisible by h. We denote Q as the restriction of K to C, which gives the classical complex (C, Q) over k. The notion of BV quantum algebra is derived from so called the BV quantization scheme. As for cultural background, the BV quantization scheme is supposed to associate a BV quantum algebra to a given "classical field theory". Then there is an art, mastered by physicists, of doing "Feynman path integrals" involving a "quantum master action functional" and a choice of "gauge fixing", on which the result of path integral is supposed to be independent.

Date: April 16, 2008

Speaker: Moira Chas, Stony Brook University

Title: "New results about Goldman's lie bracket for closed curves on a surface "

Date: January 23, 2008

Speaker: Daniel Sternheimer, Keio University, Japan

Title: "The deformation philosophy of quantization and noncommutative analogues of space-time structures"

Abstract: Deformations in physics and mathematics are part of a deformation philosophy. This philosophy was promoted in mathematical physics in joint work with Moshe Flato dating back to the 70's. One development, especially its realization on manifolds which I understand has been discussed in this seminar, is deformation quantization. This refers to deformations of commutative algebra structures into non commutative algebra structures. Another development, the deformations of algebras related to classical Lie groups leads to the so-called quantum groups with interesting connections again to topology and the topics of this seminar. One may also think of objects dual to noncommutative algebras, the so-called quantum spaces, as deformations of classical spaces, the objects dual to commutative algebras. Expressing usual geometry in terms of algebra that makes sense for noncommutative algebras leads to a rich field in mathematics called noncommutative geometry. Deforming the space-time of Einstein, Lorentz and Minkowski and its Lie group of symmetries leads to a fruitful object which together with its group of symmetries is referred as AdS or "anti de Sitter space". The study of AdS has significant physical consequences. One example is that massless particles in four dimensional space-time like photons become, in a way compatible with quantum electro dynamics, composites of massless particles in three dimensional space-time called singletons. This is part of a general correspondence between the four dimensional space time AdS theory where the geometry is related to string theory, and a three dimensional space time theory which is defined using non abelian connections and is invariant under conformal transformations. Thus the latter is a CFT, a conformally invariant quantum field theory. In physics this correspondence lead to many developments, and now there is a rich part of theoretical physics that is referred to by the name AdS/CFT correspondence. In the first part of the lecture before tea I will give an elementary introduction to these deformation ideas and survey some of these areas, always insisting on the conceptual aspects. In the second part after tea I shall attempt to develop further any points which the audience requests. We will describe an ongoing program in which anti de Sitter would be quantized in some regions related to black holes. We speculate that this could explain a universe in constant expansion and that higher mathematical structures might provide a unifying framework. Apparently higher mathematical structures such as L_infinity and A_infinity algebras are often discussed in this seminar. No prior specific knowledge will be assumed in the first part which will prepare somewhat for the second part.

Date: October 2, 2007

Speaker: Alain Connes (Collège de France)

Title: “Noncommutative geometry and physics”

Date: October 10, 2007

Speaker: Prof. Dirk Kreimer, IHES

Title: “Hochschild Cohomology in renormalizable Quantum Field Theory ”

Abstract: We review the structure of perturbative renormalization from the viewpoint of Hopf algebras in Feynman graphs. We first rederive Zimmermann's forest formula, and how it is used in quantum field theory (QFT). We try to emphasize four points:

how to obtain renormalized amplitudes in QFT

how do QFT Green functions compare to the polylogarithm

how do coideals in the Hopf algebra connect to internal symmetries of the theory

how to go beyond perturbation theory

Date: November 7, 2007

Speaker: Borya Shoikhet (IHES)

Title: "Koszul duality in deformation quantization and topological quantum field theory"