2017 Mar 09

# Colloquium: Yael Karshon (Toronto) - "Classification results in equivariant symplectic geometry"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract: I will report on some old and new classification results
in equivariant symplectic geometry,
expanding on my classification, joint with Sue Tolman,
of Hamiltonian torus actions with two dimensional quotients.
2017 Jun 15

# Colloquium: Alexander Logunov (Tel Aviv), "0,01% Improvement of the Liouville property for discrete harmonic functions on Z^2"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Let u be a harmonic function on the plane.
The Liouville theorem claims that if |u| is bounded on the whole plane, then u is identically constant.
It appears that if u is a harmonic function on a lattice Z^2, and |u| < 1 on 99,99% of Z^2,
then u is a constant function.

Based on a joint work(in progress) with L.Buhovsky, Eu.Malinnikova and M.Sodin.

2017 May 04

# Colloquium: Jozsef Solymozi (UBC) Erdos Lecture Series, "The sum-product problem"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
The incompatibility of multiplicative and additive structures in various fields and rings is an important phenomena. In this talk I will talk about a special case of it. Let us consider a finite subset of integers, A. The sum set of A is the set of pairwise sums of elements of A and the product set is the set of pairwise products. Erdős and Szemeredi conjectured that either the sum set or the product set should be large, almost quadratic in size of A. The conjecture is still open. Similar questions can be asked over any ring or field.
2015 Nov 25

# Topology & geometry: Lara Simone Suárez (HUJI), "Exact Lagrangian cobordism and pseudo-isotopy"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Abstract: Consider two Lagrangian submanifolds L, L′ in a symplectic manifold (M,ω). A Lagrangian cobordism (W;L,L′) is a smooth cobordism between L and L′ admitting a Lagrangian embedding in (([0,1]×R)×M,(dx∧dy)⊕ω) that looks like [0,ϵ)×{1}×L and (1−ϵ,1]×{1}×L′ near the boundary.
In this talk we will show that under some topological constrains, an exact Lagrangian cobordism (W;L,L′) with dim(W)>5 is diffeomorphic to [0,1]×L.
2017 Apr 20

# Basic notions: Raz Kupferman (HUJI) - A geometric framework for continuum mechanics

4:00pm to 5:15pm

Abstract:
The “geometrization" of mechanics (whether classical, relativistic or quantum) is almost as old as modern differential geometry, and it nowadays textbook material.
2017 Jun 15

# Group actions: Nir Lazarovich (ETH Zurich): Detecting sphere boundaries of hyperbolic groups

10:00am to 11:00am

We show that the boundary of a one-ended simply connected at infinity hyperbolic group with enough codimension-1 surface subgroups is homeomorphic to a sphere. By works of Markovic and Kahn-Markovic our result gives a new characterization of groups which are virtually fundamental groups of hyperbolic 3-manifolds. Joint work with B. Beeker.
2017 May 04

# Group actions: Nicolas de Saxcé (Paris 13) - Diophantine approximation and diagonal flows on the space of lattices

10:00am to 11:00am

## Location:

Ross 70
For almost every real number x, the inequality |x-p/q|<1/q^a has finitely many solutions if and only if a>2. By Roth's theorem, any irrational algebraic number x also satisfies this property, so that from that point of view, algebraic numbers and random numbers behave similarly.
2017 Jun 01

# Group actions:Lei Yang - badly approximable points on curves and unipotent orbits in homogeneous spaces

10:30am to 11:30am

We will study n-dimensional badly approximable points on curves. Given an analytic non-degenerate curve in R^n, we will show that any countable intersection of the sets of weighted badly approximable points on the curve has full Hausdorff dimension. This strengthens a previous result of Beresnevich by removing the condition on weights. Compared with the work of Beresnevich, we study the problem through homogeneous dynamics. It turns out that the problem is closely related to the study of distribution of long pieces of unipotent orbits in homogeneous spaces.
2018 Jan 22

# NT&AG: Shaul Zemel (HUJI), "Heegner Divisors on Toroidal Compactifications of Orthogonal Shimura Varieties"

2:00pm to 3:00pm

## Location:

Room 70A, Ross Building, Jerusalem, Israel
A well-known result of Borcherds yields the modularity of Heegner divisors on complex orthogonal Shimura varieties (i.e. Grassmannian quotients). These varieties are typically non-compact, and one way of completing them to compact varieties is via toroidal compactifications. However, the boundary components there also contain divisors. We show how to extend the Heegner divisors to such compactifications in such a manner that the modularity result of Borcherds still holds. This is joint work with J. Bruinier.
2016 Dec 29

# Colloquium: Jordan Ellenberg (University of Wisconsin) "The cap set problem"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
A very old question in additive number theory is: how large can a subset of Z/NZ be which contains no three-term arithmetic progression? An only slightly younger problem is: how large can a subset of (Z/3Z)^n be which contains no three-term arithmetic progression? The second problem was essentially solved in 2016, by the combined work of a large group of researchers around the world, touched off by a brilliantly simple new idea of Croot, Lev, and Pach.
2016 Mar 03

# Colloquium: Sara Tukachinsky (Hebrew University) "Counts of holomorphic disks by means of bounding chains"

3:30pm to 4:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Over a decade ago, Welschinger defined invariants of real symplectic manifolds of complex dimensions 2 and 3, which count pseudo-holomorphic disks with boundary and interior point constraints. Since then, the problem of extending the definition to higher dimensions has attracted much attention.
2015 Nov 12

# Colloquium: Michael Krivelevich (Tel Aviv), "Positional games"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Title: Positional games
Positional games are a branch of combinatorics, researching a variety of two-player games, ranging from popular recreational games such as Tic-Tac-Toe and Hex, to purely abstract games played on graphs and hypergraphs. It is closely connected to many other combinatorial disciplines such as Ramsey theory, extremal graph and set theory, probabilistic combinatorics, and to computer science.
2016 May 05

# Colloquium: Daniel Wise (McGill) "The Cubical Route to Understanding Groups"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract:
Cube complexes have come to play an increasingly central role within geometric group theory, as their connection to right-angled Artin groups provides a powerful combinatorial bridge between geometry and algebra. This talk will introduce nonpositively curved cube complexes, and then describe the developments that have recently culminated in the resolution of the virtual Haken conjecture for 3-manifolds, and simultaneously dramatically extended our understanding of many infinite groups.
2016 Dec 15

# Colloquium: Cy Maor (Toronto) "Asymptotic rigidity of manifolds"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Liouville's rigidity theorem (1850) states that a map $f:\Omega\subset R^d \to R^d$ that satisfies $Df \in SO(d)$ is an affine map. Reshetnyak
(1967) generalized this result and showed that if a sequence $f_n$
satisfies $Df_n \to SO(d)$ in $L^p$, then $f_n$ converges to an affine
map.
In this talk I will discuss generalizations of these theorems to mappings
between manifolds and sketch the main ideas of the proof (using techniques
from the calculus of variations and from harmonic analysis).
2016 Jan 07

# Colloquium: Peter Ozsváth (Princeton), "Zabrodsky Lectures: Knot Floer homology"

3:30pm to 4:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract: Knot Floer homology is an invariant for knots, defined using methods from symplectic geometry. This invariant contains topological information about the knot, such as its Seifert genus; it can be used to give bounds on the unknotting number; and it can be used to shed light on the structure of the knot concordance group. I will outline the construction and basic properties of knot Floer. Knot Floer homology was originally defined in collaboration with Zoltan Szabo, and independently by Jacob Rasmussen.