2014 Mar 20

2:30pm to 3:30pm

2015 Oct 22

# Colloquium: Nir Avni (Northwestern), "Counting points and counting representations"

2:30pm to 3:30pm

Title: Counting points and counting representations
Abstract:
I will talk about the following questions:
1) Given a system of polynomial equations with integer coefficients, how many solutions does it have in the ring Z/N?
2)​ Given a polynomial map f:R^a-->R^b and a smooth, compactly supported measure m on R^a, does the push-forward of m by f have bounded density?
3) Given a lattice in a higher rank Lie group (say, SL(n,Z) for n>2). How many d-dimensional representations does it ​have?
2014 May 22

2:30pm to 3:30pm

2014 Jan 16

2:30pm to 3:30pm

2014 Feb 27

2:30pm to 3:30pm

2013 Nov 14

2:30pm to 3:30pm

2017 Sep 14

# Colloquium: Kate Juschenko (Northwestern University) - "Cycling amenable groups and soficity"

2:30pm to 3:30pm

## Location:

IIAS hall, Hebrew University Jerusalem
I will give introduction to sofic groups and discuss a possible strategy towards finding a non-sofic group. I will show that if the Higman group were sofic, there would be a map from Z/pZ to itself, locally like an exponential map, satisfying a rather strong recurrence property. The approach to (non)-soficity is based on the study of sofic representations of amenable subgroups of a sofic group. This is joint work with Harald Helfgott.
2016 Jan 10

# Zabrodsky lecture series: Peter Ozsváth (Princeton) "Computational aspects of knot Floer homology"

4:00pm to 5:00pm

## Location:

Ross 70A
Abstract: The original construction uses the theory of pseudo-holomorphic curves. In this lecture, I will describe an explicit combinatorial algorithm for computing knot Floer homology in terms of grid diagrams. In this lecture, I will describe joint work with Ciprian Manolescu, Sucharit Sarkar, Zoltan Szabo, and Dylan Thurston.
2017 Jun 21

2:00pm to 2:50pm

Ross 70A.
Abstract:
2017 Sep 12

# T&G: Liat Kessler (Cornell and Oranim), Extending Homologically trivial symplectic cyclic actions to Hamiltonian circle actions

12:00pm to 1:00pm

## Location:

Ross Building Room 70A
We ask whether every homologically trivial cyclic action on a symplectic four-manifold extend to a Hamiltonian circle action. By a cyclic action we mean an action of a cyclic group of finite order; it is homologically trivial if it induces the identity map on homology. We assume that the manifold is closed and connected. In the talk, I will give an example of a homologically trivial symplectic cyclic action on a four-manifold that admits Hamiltonian circle actions, and show that is does not extend to a Hamiltonian circle action.
2016 Jun 15

# Topology & geometry, Ezra Getzler (Northwestern University), "The derived Maurer-Cartan locus"

2:00pm to 3:35pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Abstract:
Derived algebraic geometry is a nonlinear analogue of homological algebra, in which one keeps track of syzygies among the relations among the defining equations of a variety, and higher analogues. It has important applications to intersection theory and enumerative geometry.
2017 May 23

# Topology & Geometry Seminar: Adina Gamse (University of Toronto), "Vanishing relations in the cohomology of the moduli space of parabolic bundles".

1:00pm to 1:50pm

## Location:

Ross A70.

Abstract: Let \Sigma be a compact connected oriented 2-manifold of genus g , and let p be a point on \Sigma. We define a space S_g(t) consisting of certain irreducible representations of the fundamental group of \Sigma - { p } , modulo conjugation by SU(N).
2017 Aug 09

# T&G: Peter Ozsvath (Princeton), Bordered methods in knot Floer homology

12:00pm to 1:00pm

## Location:

Room 70A, Ross Building, Jerusalem, Israel
Knot Floer homology is an invariant for knots in the three-sphere defined using methods from symplectic geometry. I will describe a new algebraic formulation of this invariant which leads to a reasonably efficient computation of these invariants. This is joint work with Zoltan Szabo.
2016 Jan 11

# Zabrodsky lecture series: Peter Ozsváth (Princeton) "Bordered Floer homology"

12:00pm to 1:00pm

## Location:

Ross 70A
Abstract: Bordered Floer homology is an invariant for three-manifolds with boundary, defined in collaboration with Robert Lipshitz and Dylan Thurston. The invariant associates a DG algebra to a parameterized surface, and a module over that algebra to a three-manifold with boundary. I will explain how methods from bordered Floer homology can be used to give a tidy description of knot Floer homology. This is joint work with Zoltan Szabo.
2015 Dec 17

# Groups & dynamics: Robert Hough (IAS) - Mixing and cut-off on cyclic groups

12:00pm to 1:00pm

## Location:

Einstein 110
Consider a sequence of random walks on $\mathbb{Z}/p\mathbb{Z}$ with symmetric generating sets $A= A(p)$. I will describe known and new results regarding the mixing time and cut-off. For instance, if the sequence $|A(p)|$ is bounded then the cut-off phenomenon does not occur, and more precisely I give a lower bound on the size of the cut-off window in terms of $|A(p)|$. A natural conjecture from random walk on a graph is that the total variation mixing time is bounded by maximum degree times diameter squared.