2018 May 07

# HD-Combinatorics: Special day on group stability

(All day)

## Location:

Eilat Hall, Feldman Building, Givat Ram

This special day is part of several Mondays that will be dedicated to stability in group theory

09:00 - 11:00 Alex Lubotzky, "Group stability and approximation"

14:00 - 16:00 Lev Glebsky, "Stability and second cohomology"
2018 Jun 26

# Dynamics Lunch: Jasmin Matz (Huji) ״Distribution of periodic orbits of the horocycle flow״

12:00pm to 1:00pm

## Location:

Manchester lounge
An old result of Hedlund tells us that there are no closed orbits for the horocycle flow on a compact Riemann surface M. The situation is different if M is non-compact in which case there is a one-parameter family of periodic orbits for every cusp of M. I want to talk about a result by Sarnak concerning the distribution of the such orbits in each of these families when their length goes to infinity. It turns out that these orbits become equidistributed in M and the rate of convergence can in fact be quantified in terms of spectral properties of the Eisenstein series on M.
2018 May 03

# Special Talk : W Chacholski (Copenhagen) - "What is persistence?"

## Lecturer:

W Chacholski (Copenhagen)
1:15pm to 2:00pm

Ross 63
2018 May 03

# Basic Notions - Alex Lubotzky: "Group stability and approximation"

4:00pm to 5:30pm

## Location:

Ross 70
An old problem (Going back to Turing, Ulam and others) asks about the "stability" of solutions in some algebraic contexts. We will discuss this general problem in the context group theory: Given an "almost homomorphism" between two groups, is it close to a homomorphism?
2018 May 22

# Barak Weiss (TAU): New examples for the horocycle flow on the moduli space of translation surfaces

2:15pm to 3:15pm

A longstanding open question concerning the horocycle flow on moduli space of translation surfaces, is whether one can classify the invariant measures and orbit-closures for this action. Related far-reaching results of Eskin, Mirzakhani and Mohammadi indicated that the answer might be positive. However, in recent work with Jon Chaika and John Smillie, we find unexpected examples of orbit-closures; e.g. orbit closures which are not generic for any measure, and others which have fractional Hausdorff dimension. Such examples exist even in genus 2.
2018 Jun 18

# Combinatorics -- Erdos lecture cancelled; instead (NOTE THE TIME!) :

10:30am to 12:30pm

## Location:

IIAS, Eilat hall, Feldman bldg (top floor), Givat Ram
Speaker: Ilan Newman and Yuri Rabinovich, U.Haifa
Title: Sparsifiers - Part I (Part II from 2pm to 4pm, same day and place)
Abstract:
Time permitting, we plan to discuss the following topics (in this order):
1.
* Additive Sparsification and VC dimension
* Multiplicative Sparsification
* Examples: cut weights, cut-dimension of L_1 metrics, general metrics,
and their high-dimensional analogues
2.
* Multiplicative Sparsification and Triangular Rank;
* Karger-Benczur sparsification of cuts weights
2018 May 28

# Combinatorics: Daniel Jerison (TAU) "Random walks on sandpile groups"

11:00am to 12:30pm

## Location:

IIAS, Eilat hall, Feldman bldg, Givat Ram
Speaker: Daniel Jarison,  TAU

Title: Random walks on sandpile groups

Abstract:
2018 Jun 25

# Combinatorics: Roman Glebov (HU) "Perfect Matchings in Random Subgraphs of Regular Bipartite Graphs"

11:00am to 12:30pm

## Location:

IIAS, room 130, Feldman bldg, Givat Ram
Speaker: Roman Glebov (HU)
Title: Perfect Matchings in Random Subgraphs of Regular Bipartite Graphs
Abstract:
Consider the random process in which the edges of a graph $G$ are
added one by one in a random order. A classical result states that if
$G$ is the complete graph $K_{2n}$ or the complete bipartite graph
$K_{n,n}$, then typically a perfect matching appears at the moment at
which the last isolated vertex disappears. We extend this result to
arbitrary $k$-regular bipartite graphs $G$ on $2n$ vertices for all
$k=\Omega(n)$.
2018 Jun 11

# Combinatorics: Chris Cox (CMU) "Nearly orthogonal vectors"

11:00am to 12:30pm

## Location:

IIAS, Eilat hall, Feldman bldg, Givat Ram
Speaker: Chris Cox, CMU
Title -- Nearly orthogonal vectors

2018 Jun 05

# Dynamics Lunch: Hagai Lavner (Huji)

12:00pm to 1:00pm

## Location:

Manchester lounge