EGGG

East German Groups and Geometry

East German Groups and Geometry is a seminar series designed to bring together people in eastern Germany working in (geometric) group theory and related fields. We aim to meet for one afternoon three times per year.

Upcoming Meetings

May 7th 2026, University of Potsdam

Our next meeting takes place on 7 May at the University of Potsdam, in room 2.22 of the Institute of Mathematics (House 9, Campus Golm). There will be three afternoon talks from 13:15 until 17:00. We are pleased to let you know that Anna Cascioli (University of Münster), Waltraud Lederle (University of Bielefeld) and Piotr Mizerka (Adam Mickiewicz University in Poznań) have agreed to give talks. We also plan to have dinner at 18:00 in downtown Potsdam.

If you would like to join, please fill out the sign-up form.

Schedule

Compact Invariant Random Subgroups [13:15-14:15]

Waltraud Lederle, University of Bielefeld

An IRS is a conjugacy-invariant probability measure on the space of subgroups of a locally compact group. We are interested in those IRS that give full measure to the set of compact subgroups. This talk is about what we know about those, how it connects to the structure theory of locally compact groups, and what we would still like to figure out.

Joint with Tal Cohen, Helge Glöckner and Gil Goffer.

Stationary boundaries on the space of amenable subgroups and C*-simplicity [14:25-15:25]

Anna Cascioli, University of Münster

Every countable group acts on its space of subgroups, and the induced dynamics on the space of amenable subgroups encode structural features of the group. We study stationarity for this action and give a sufficient condition for the existence of a probability measure μ on G that admits a non-trivial μ-boundary modeled on the space of amenable subgroups of G. This leads to non-uniqueness of stationary measures and connects to C*-simplicity, a property that naturally arises in the study of unitary representations of a group. The criterion applies to wreath products and to Thompson’s group F. This is joint work with Martín Gilabert Vio and Eduardo Silva.

Coffee break [15:25-16:00]

Non-vanishing of group cohomology of SL(n,Z) in the rank [16:00-17:00]

Piotr Mizerka, Adam Mickiewicz University

It has been recently shown by Bader and Sauer that the cohomology of SL(n,Z) with coefficients in orthogonal representations without non-trivial invariant vectors vanishes below the rank. We show that for n=3 and n=4 their result is sharp: we indicate specific representations for which SL(n,Z) possesses non-trivial cohomology in the rank. We apply the Steinberg duality which allows us to compute the group cohomologies of interest by means of a specific model of a symmetric space. The key idea is to translate such computations to calculations stemming from group rings. The latter could be accomplished with the help of computers. This is the joint work with B. Brück, S. Hughes, and D. Kielak.

Previous meetings

February 2nd 2026, Technische Universität Berlin

The inaugural meeting of EGGG will take place on the afternoon of Monday 2nd February 2026 at TU Berlin in room ER 164 (see the campus map). The titles and abstracts are below. The event will begin at 13:30, with the first talk at 13:45. Afterwards, we will go for dinner at Saigon Green at 17:45.

If you wish to attend, please fill out the sign up form. Please complete the form by the end of Thursday 29th January if you wish to join for dinner.

Schedule

Outer automorphism groups of Leary–Minasyan groups [13:45-14:45]
Motiejus Valiunas, University of Wrocław

Baumslag–Solitar groups, i.e. commensurating HNN-extensions of Z, have served as counterexamples to a variety of claims throughout the years – giving examples of finitely presented groups that are not Hopfian, or Hopfian but not residually finite, just to name a couple. More recently, the outer automorphism groups Out(G) of Baumslag–Solitar groups G have been studied, giving examples of such Out(G) that are not finitely generated. Following the tradition of studying this “bad” behaviour of Baumslag–Solitar-like groups, one may analyse the properties of Leary–Minasyan groups, i.e. commensurating HNN-extensions of Z^n; these have provided the first examples of CAT(0) groups that are not biautomatic. In the talk, I will describe Out(G) for Leary–Minasyan groups G, and give examples (to my knowledge, first such) of finitely presented metabelian groups with non-finitely generated Out. This is joint work with Naomi Andrew and Sam Hughes.

Stable subgroups of graph products [14:45-15:45]
Marie Trin, Max Planck Institute Leipzig

Stable subgroups of non-hyperbolic groups have been introduced by Durham and Taylor as a well defined notion of convexity in non-hyperbolic groups. For different examples of groups, stable subgroups are characterized through a well known action on a hyperbolic space. We will describe such characterizations of stable subgroups in RAAGs and mapping class groups and will see what are the analogs for graph products. This is based on joint work with S. H. Balasubramanya, M. Chesser, A. Kerr and J. Mangahas.

Coffee break [15:45-16:30]

Right-angled Coxeter groups: structure of their Hecke (operator) algebras [16:30-17:30]
Sven Raum, University of Potsdam

In this talk I will survey work on the structure of Hecke algebras and Hecke operator algebras associated with right-angled Coxeter groups done by various people over the past decade. I will assume a minimal background on Coxeter groups, but none on Hecke algebras or operator algebras. Results we discuss concern:

1) description of the centre and cocentre of Hecke algebras;

2) decomposition of Hecke von Neumann algebras into simple direct summands (aka factor decomposition);

3) simplicity of Hecke C*-algebras; and

4) calculation of algebraic and non-commutative geometry invariants of Hecke C*-algebras.

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