EGGG

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

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. In the evening we will go for dinner nearby. 

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.

Outer automorphism groups of Leary–Minasyan groups
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
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.

Right-angled Coxeter groups: structure of their Hecke (operator) algebras  
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.