Oct 21, In Building 34, room 14

Intermediate subalgebras of commutative crossed products of discrete group actions.Online

Tattwamasi Amrutam (BenGurion University)

In this talk, we shall focus our attention on intermediate subalgebras of $C(X)\rtimes_r\Gamma$ (and $L^{\infty}(X,\nu)\ltimes\Gamma$). We begin by describing the construction of the commutative crossed product $C(X)\rtimes_r\Gamma$ and how the group contributes to its structure. We shall talk about various (generalized) averaging properties in this context. As a first application, we will show that every intermediate $C^*$subalgebra $\mathcal{A}$ of the form $C(Y)\rtimes_r\Gamma\subseteq\mathcal{A}\subseteq C(X)\rtimes_r\Gamma$ is simple for an inclusion $C(Y)\subset C(X)$ of minimal $\Gamma$spaces whenever $C(Y)\rtimes_r\Gamma$ is simple. We shall also show that, for a large class of actions of $C^*$simple groups $\Gamma\curvearrowright X$, including nonfaithful action of any hyperbolic group with trivial amenable radical, every intermediate $C^*$algebra $\mathcal{A}$, $C_r^*(\Gamma)\subset \mathcal{A}\subset C(X)\rtimes_r\Gamma$ is a crossed product of the form $C(Y)\rtimes_r\Gamma$, $C(Y)\subset C(X)$ is an inclusion of $\Gamma$$C^*$algebras.

Nov 4, In Building 34, room 14

Bohr Chaos and Invariant MeasuresOnline

Matan Tal (The Hebrew University)

A topological dynamical system is said to be Bohr chaotic if for any bounded sequence it possesses a continuous function that correlates with the sequence when evaluated along some orbit. The theme of the lecture will be the relation of this property to an abundance of invariant measures of the system.

Nov 11

Allosteric actions of surface groupsOnline

Matthieu Joseph (École normale supérieure de Lyon)

In a recent work, I introduced the notion of allosteric actions: a minimal action of a countable group on a compact space, with an ergodic invariant measure, is allosteric if it is topologically free but not essentially free. In the first part of my talk I will explain some properties of allosteric actions, and their links with Invariant Random Subgroups (IRS) and Uniformly Recurrent Subgroups (URS). In the second part, I will explain a recent result of mine: the fundamental group of a closed hyperbolic surface admits allosteric actions.

Nov 18, In Building 34, room 14

Introduction to bounded cohomologyOnline

Anton Hase (BenGurion University)

While there are earlier works on bounded cohomology, the topic was popularized by Gromov in 1982. In this introductory talk, we will give definitions of bounded cohomology of discrete groups with trivial coefficients. We will interpret bounded cohomology in low degrees in terms of quasimorphisms and central extensions. Then we will mention a few examples of how bounded cohomology has proved useful in applications, before concentrating on the classification of circle actions

Nov 25

TBAOnline

Trip to the desert of the PET seminar group!!


Dec 2

Stabilizers in group Cantor actions and measuresOnline

Olga Lukina (University of Vienna)

Given a countable group G acting on a Cantor set X by transformations preserving a probability measure, the action is essentially free if the set of points with trivial stabilizers has a full measure. In this talk, we consider actions where no point has a trivial stabilizer, and investigate the properties of the points with nontrivial holonomy. We introduce the notion of a locally nondegenerate action, and show that if an action is locally nondegenerate, then the set of points with trivial holonomy has full measure in X. We discuss applications of this work to the study of invariant random subgroups, induced by actions of countable groups. This is joint work with Maik Gröger.

Dec 9

Symbolic discrepancy and Pisot dynamicsOnline

Valérie Berthé (Université de Paris)

Discrepancy is a measure of equidistribution for sequences of points. A bounded remainder set is a set with bounded discrepancy, that is, the number of times it is visited differs by the expected time only by a constant. We discuss dynamical, symbolic, and spectral approaches to the study of bounded remainder sets for Kronecker sequences. We consider in particular discrepancy
in the setting of symbolic dynamics and we discuss the existence of bounded remainder sets for some families of zero entropy subshifts.
Note that bounded discrepancy has also to do with the notion of bounded displacement to a lattice in the context of Delone sets. We focus on the case of Pisot parameters for toral translations and then show how to construct symbolic codings in terms of multidimensional continued fraction
algorithms.
This is joint work with W. Steiner and J. Thuswaldner.

Dec 16

Linnik’s basic lemma with uniformity over the base fieldOnline

Andreas Wieser (The Hebrew University)

Long periodic geodesics on the unit tangent bundle of the modular surface are not necessarily equidistributed. However, there is a natural way to group finitely many geodesics together so that the soobtained unions do equidistribute. This theorem (in this instance going back to Duke ‘88) is very well studied nowadays. In the talk, we discuss a dynamical approach due to Linnik through what is nowadays called Linnik’s basic lemma (providing in particular an entropy lower bound). We present here a new proof of Linnik’s basic lemma based on geometric invariant theory. This is joint work with Pengyu Yang.

Dec 23

A Probabilistic Algorithm for Vertex CoverOnline

Shaked Mamana (BGU)

The Vertex Cover Problem is the optimization problem of finding a vertex cover V_c of minimal cardinality in a given graph. It is a classic NPhard problem, and various algorithms have been suggested for it.
In this talk, we will start with a basic algorithm for solving the problem. Using a probabilistic idea, we use it to develop an improved algorithm. The algorithm is greedy; at each step it adds to the cover a vertex such that the expected cover size, if we continue randomly after this step, is minimal. We will study the new algorithm theoretically and empirically, and present simulations that compare its performance to that of some algorithms of a similar nature.

Dec 30

Nonclassifiability of ergodic flows up to time changeOnline

Philipp Kunde (Universität Hamburg )


Jan 6, In Building 34, room 14

The Ramanujan Machine: Polynomial Continued Fraction and Irrationality MeasureOnline

Nadav BenDavid (BenGurion University)

Apéry’s proof of the irrationality of ζ(3) used a specific linear recursion that formed a Polynomial Continued Fraction (PCF). Similar PCFs can prove the irrationality of other fundamental constants such as 𝜋 and e. However, in general, it is not known which ones create useful Diophantine approximations and under what conditions they can be used to prove irrationality.
Here, we will present theorems and general conclusions about Diophantine approximations created from polynomial recursions. Specifically, we generalize Apéry’s work from his particular choice of PCF to any general PCF, finding the conditions under which a PCF can be used to prove irrationality or to provide an efficient Diophantine approximation. We further propose new conjectures about Diophantine approximations based on PCFs. Our study may contribute to ongoing efforts to answer open questions such as the proof of the irrationality of the Catalan constant or of values of the Riemann zeta function (e.g., ζ(5)).

Jan 13

Substitutions on compact alphabetsOnline

Dan Rust (The Open University (UK))

Substitutions and their subshifts are classical objects in symbolic dynamics representing some of the most wellstudied and ‘simple’ aperiodic systems. Classically they are defined on finite alphabets, but it has recently become clear that a systematic study of substitutions on infinite alphabets is needed. I’ll introduce natural generalizations of classical concepts like legal words, repetitivity, primitivity, etc. in the compact Hausdorff setting, and report on new progress towards characterising unique ergodicity of these systems, where surprisingly, primitivity is not sufficient. As PerronFrobenius theory fails in infinite dimensions, more sophisticated technology from the theory of positive (quasicompact) operators is employed. There are still lots of open questions, and so a groundlevel introduction to these systems will hopefully be approachable and stimulating.
This is joint work with Neil Mañibo and Jamie Walton.
