2017 Nov 23

# Shmuel Weinberger, "Introduction to quantitative topology (continued)"

9:00am to 10:00am

## Location:

Room 130, Feldman building (IIAS), Givat Ram
2017 Dec 11

# HD-Combinatorics: Izhar Oppenheim, "New Constructions of local spectral high dimensional expanders"

2:00pm to 4:00pm

## Location:

Eilat Hall, Feldman Building (IIAS), Givat Ram
Abstract:
2017 Nov 06

# High Dimensional Expanders and Group Stability, Alex Lubotkzy

9:00am to 11:00am

## Location:

Room 130
In the first talk we gave a brief outline of the contents of the course. In the rest of the semester we will get deeper into some topics. In the coming lecture ( and the next one) we will discuss Kazhdan property T and its connections with expanders and with first cohomology groups. No prior knowledge will be assumed.
2017 Nov 06

2:00pm to 4:00pm

Room 130
2018 Jan 08

# HD-Combinatorics: Amnon Ta-Shma, "Bias samplers and reducing overlap in random walks over graphs"

2:00pm to 4:00pm

Abstract:

The expander Chernoff bound states that random walks over expanders are good samplers, at least for a certain range of parameters. In this talk we will be interested in “Parity Samplers” that have the property that for any test set, about half of the sample sets see the test set an *even* number of times, and we will check whether random walks over expanders are good parity samplers. We will see that:

1. Random walks over expanders fare quite well with the challenge, but,
2. A sparse Random complex does much better.
2017 Oct 23

# Ori Parzanchevski: From Ramanujan graphs to Ramanujan Complexes (course)

4:00pm to 6:00pm

## Location:

Feldman building, Eilat Hall
2017 Nov 27

# HD-Combinatorics: Irit Dinur, "PCPs and high dimensional expansion"

2:00pm to 4:00pm

## Location:

Room 130, Feldman Building (IIAS), Givat Ram
The "PCP theorem" says that problems in NP are hard in a robust or stable way. I will give a brief intro to PCPs (and explain the acronym) and then try to outline a proof of the PCP theorem based on "agreement expansion" which is a form of high dimensional expansion. My aim is to show how high dimensional expansion is inherently present in PCP type questions.
2017 Dec 04

# HD-Combinatorics: Tali Kaufman, "High dimensional expanders imply PCP-agreement expansion"

2:00pm to 4:00pm

## Location:

Room 130, Feldman Building (IIAS), Givat Ram
Abstract: I will introduce the notion of (PCP)-agreement expansion which is an important building block in PCPs constructions. I will then show that a high dimensional expanders imply PCP-agreement expanders. based on Joint work with Irit Dinur
2017 Sep 05

# IIAS Seminar: Tatiana Nagnibeda - Infinite Ramanujan graphs and completely dissipative actions

4:00pm to 5:00pm

## Location:

Math room 209
Speaker : Tatiana Nagnibeda (University of Geneva) Abstract: The definition of a Ramanujan graph extends naturally to infinite graphs: an infinite graph is Ramanujan if its spectral radius is not larger than (and hence equal to) the spectral radius of its universal covering tree. As with infinite families of finite graphs, it is interesting and non-trivial to understand, how much Ramanujan graphs resemble trees. I will discuss some results in this direction obtained in a joint work with Vadim Kaimanovich, by investigating ergodic properties of boundary actions of free groups.
2017 Dec 18

# HD-Combinatorics: Steven Damelin, "Approximate and exact alignment of data, extensions and interpolation in R^D--parts"

2:00pm to 4:00pm

## Location:

Sprinzak Building, Room 28
Speaker: Steven Damelin (The American Mathematical Society) Abstract: A classical problem in geometry goes as follows. Suppose we are given two sets of $D$ dimensional data, that is, sets of points in $R^D$. The data sets are indexed by the same set, and we know that pairwise distances between corresponding points are equal in the two data sets. In other words, the sets are isometric. Can this correspondence be extended to an isometry of the ambient Euclidean space? In this form the question is not terribly interesting; the answer has long known
2017 Nov 20

# Andreas Thom, "C*-algebras and group theory"

9:00am to 11:00am

## Location:

Room 130, Feldman Building (IIAS), Givat Ram
2017 Nov 27

# Andreas Thom, "C*-algebras and group theory (continued)"

9:00am to 11:00am

## Location:

Room 130, Feldman Building (IIAS), Givat Ram
2017 Dec 25

# HD-Combinatorics: Shai Evra, "Bounded degree high dimensional expanders"

2:00pm to 4:00pm

In the recent theory of high dimensional expanders, the following open problem was raised by Gromov: Are there bounded degree high dimensional expanders? For the definition of high dimensional expanders, we shall follow the pioneers of this field, and consider the notions of coboundary expanders (Linial-Meshulam) and topological expanders (Gromov). In a recent work, building on an earlier work of Kaufman-Kazhdan-Lubotzky in dimension 2, we were able to prove the existence of bounded degree expanders according to Gromov, in every dimension.
2017 Nov 20

# Leonard Schulman, "Analysis of a Classical Matrix Preconditioning Algorithm"

2:00pm to 3:00pm

## Location:

Room 130, Feldman Building, Givat Ram
There are several prominent computational problems for which simple iterative methods are widely preferred in practice despite an absence of runtime or performance analysis (or "worse", actual evidence that more sophisticated methods have superior performance according to the usual criteria). These situations raise interesting challenges for the analysis of algorithms. We are concerned in this work with one such simple method: a classical iterative algorithm for balancing matrices via scaling transformations. This algorithm, which goes back to Osborne and
2018 Jan 10

# Logic Seminar - Alex Lubotzky - "First order rigidity of high-rank arithmetic groups"

11:00am to 1:00pm

## Location:

Ross 63
The family of high rank arithmetic groups is a class of groups playing an important role in various areas of mathematics. It includes SL(n,Z), for n>2 , SL(n, Z[1/p] ) for n>1, their finite index subgroups and many more. A number of remarkable results about them have been proven including; Mostow rigidity, Margulis Super rigidity and the Quasi-isometric rigidity.