2016 Jun 16

# Amitsur Algebra: Gili Golan, "The generation problem in Thompson group F"

12:00pm to 1:15pm

## Location:

Manchester Building (room 209), Jerusalem, Israel
We show that the generation problem in Thompson group F is decidable, i.e., there is an algorithm which decides if a finite set of elements of F generates the whole F. The algorithm makes use of the Stallings 2-core of subgroups of F, which can be defined in an analogue way to the Stallings core of subgroups of a free group. An application of the algorithm shows that F is a cyclic extension of a group K which has a maximal elementary amenable subgroup B. The group B is a copy of a subgroup of F constructed by Brin.
2016 Apr 07

# Amitsur Algebra: Ayala Byron (HUJI), "Definable fields in the free group"

12:00pm to 1:15pm

## Location:

Manchester Building (room 209), Jerusalem, Israel
Abstract: In the early 2000s Sela proved that all non-abelian free groups share a common first-order theory. Together with R. Sklinos, we use tools developed in his work to show that no infinite field is definable in this theory. In this talk we will survey the line of proof for a formal solution theorem for a simple sort of definable sets, that have a structure of a hyperbolic tower, and use it to characterize definable sets that do not carry a definable structure of an abelian group.
2018 Apr 25

# Analysis Seminar: Latif Eliaz "The Essential Spectrum of Schroedinger Operators on Graphs"

12:00pm to 1:00pm

## Location:

Room 70, Ross Building

It is known that the essential spectrum of aSchrödinger operator H on\ell^2(\mathbb{N})  is equal to the union of the spectra of right limits ofH. The naturalgeneralization of this relation to \mathbb{Z}^n  is known to hold as well.In this talk we study thepossibility of generalizing this characterization of \sigma_{ess}(H)  tographs. We show that the general statement fails, while presenting natural families of models where it still holds.

2018 Jan 30

# Action Now Seminar: Menny Aka (ETH), "On rational planes in four-dimensional space and their associated lattices"

4:00pm to 5:00pm

## Location:

IIAS, HUJI, Feldman Building, Room 130
2018 Jan 30

# Action Now Seminar: Tammy Ziegler (HUJI), "Dynamics, arithmetic progressions and approximate cohomology"

10:00am to 11:00am

## Location:

IIAS, HUJI, Feldman Building, Room 130
2018 Jan 30

# Action Now Seminar: Weikun He (HUJI), "Discretized sum-product estimates in higher dimension"

2:30pm to 3:30pm

## Location:

IIAS, HUJI, Feldman Building, Room 130
2018 Jan 30

# Action Now Seminar: Oren Becker (HUJI), "Stability and Invariant Random Subgroups"

11:30am to 12:30pm

## Location:

IIAS, HUJI, Feldman Building, Room 130
2018 Apr 12

# Colloquium: Ron Peretz (Bar Ilan) - "Repeated Games with Bounded Memory - the Entropy Method"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract:
In the past two decades the entropy method has been successfully employed in the study of repeated games. I will present a few results that demonstrate the relations between entropy and memory. More specifically: a finite game is repeated (finitely or infinitely) many times. Each player $i$ is restricted to strategies that can recall only the last $k_i$ stages of history. The goal is to characterize the (asymptotic) set of equilibrium payoffs. Such a characterization is available for two-player games, but not for three players or more.
Related papers:
2018 Jan 21

# NT&AG: Daniel Disegni (University of Paris-Sud 11), On the p-adic Bloch-Kato conjecture for Hilbert modular forms

3:00pm to 4:00pm

## Location:

Room 70A, Ross Building, Jerusalem, Israel
The Birch and Swinnerton-Dyer conjecture predicts that the group of rational points on an elliptic curve E over Q has rank equal to the order of vanishing of the L-function of E. Generalisations of this conjecture to motives M were formulated by Belinson and Bloch-Kato. I will explain a proof of a version of the Bloch-Kato conjecture in p-adic coefficients, when M is attached to a p-ordinary Hilbert modular form of any weight and the order of vanishing is 1.
2017 Mar 02

# Group actions seminar: David El-Chai Ben Ezra (HUJI) - The congruence subgroup problem for automorphism groups of  free meta-abelian groups

10:30am to 11:30am

2017 Nov 02

# Group actions: Remi Coulon (Rennes) - Growth gap in hyperbolic groups and amenability

10:30am to 11:30am

## Location:

hyperbolic groups and amenability
(joint work with Françoise Dal'Bo and Andrea Sambusetti)
Given a finitely generated group G acting properly on a metric space X,
the exponential growth rate of G with respect to X measures "how big"
the orbits of G are. If H is a subgroup of G, its exponential growth
rate is bounded above by the one of G. In this work we are interested in
the following question: what can we say if H and G have the same
exponential growth rate? This problem has both a combinatorial and a
geometric origin. For the combinatorial part, Grigorchuck and Cohen
2017 Apr 27

# Group actions: Yair Glasner (BGU) - On Highly transitive permutation representations of groups.

10:30am to 11:30am

## Location:

Ross 70
Abstract: A permutation representation of a group G is called highly transitive if it is transitive on k-tuples of points for every k. Until just a few years ago groups admitting such permutation representations were thought of as rare. I will focus on three rather recent papers: G-Garion, Hall-Osin, Gelander-G-Meiri (in preparation) showing that such groups are in fact very common.
2018 May 10

# Groups & dynamics: Sanghoon Kwon (Kwandong University) - A combinatorial approach to the Littlewood conjecture in positive characteristic

10:30am to 11:30am

## Location:

Ross 70
The Littlewood conjecture is an open problem in simultaneous Diophantine approximation of two real numbers. Similar problem in a field K of formal series over finite fields is also still open. This positive characteristic version of problem is equivalent to whether there is a certain bounded orbit of diagonal semigroup action on Bruhat-Tits building of PGL(3,K).
2018 Feb 19

# HD-Combinatorics Special Day: Error correcting codes and high dim complexes (organised by Gilles Zemor)

(All day)

## Location:

IIAS, Feldman Building, Givat Ram

10:00 - 11:00 Gilles Zemor. Background on quantum computing and coding.
11:30 - 12:30 Gilles Zemor. Quantum stabilizer codes and quantum LDPC codes.
14:00 - 15:00. Dorit Aharonov. Quantum locally testable codes.
15:15 - 16:15. Gilles Zemor. Quantum LDPC codes and high-dimensional expanders.
2018 Jan 21

# Game Theory & Math Economics: Benny Moldovanu, University of Bonn "A Theory of Actions with Endogenous Valuations" (joint work with Alex Gershkov and Philipp Strack)

4:00pm to 5:00pm

## Location:

Elath Hall, 2nd floor, Feldman Building, Edmond Safra Campus