Group Theory and Friends Oberseminar

This seminar is organized by the Applied Algebra Group at TU Berlin. Anyone interested is welcome to join us. We meet roughly every second week. A detailed schedule can be seen below and will be updated throughout the semester.

 Upcoming Events

October 27, 2026 14:00 MA 366: Peter Buergisser (Berlin)

Title: On the number of real zeros of structured random polynomials

Abstract: A famous result by Khovanskii (1980) bounds the number of positive real zeros of polynomial systems of equations in terms of their sparsity (number of monomials). This upper bound is huge and appears to be very far from optimality, but very little is known.

I will discuss two results stating that structured (systems of) random polynomials typically only have few real zeros.

The first result is on random fewnomials: it says that a system of polynomials in n variables with a prescribed set of t terms and independent centered Gaussian coefficients has an expected number of positive real zeros bounded by 2 times the binomial coefficient of t choose n.

The second result is on Koiran's Real Tau Conjecture, which claims that the number of real zeros of a sum of m products of k real sparse univariate polynomials, each with a fixed set of at most t terms, is bounded by a polynomial in m, k, and t. The Real Tau Conjecture implies Valiant's Conjecture VP ≠ VNP. We have confirmed the conjecture on average: if the coefficients in these structured polynomials are independent standard Gaussians, then the expected number of real zeros is bounded by a constant times m times k squared times t.

The proofs are based on the Rice formula and methods from integral geometry.

November 27th 2026, Max Planck Institute for Mathematics in the Sciences

Our next EGGG meeting will take place on Friday 27th November at the Max Planck Institute for Mathematics in the Sciences (MPI MiS), Leipzig, in the Leibniz Saal (see the building map). The event will begin at 13:30, with the first talk at 13:45, and the event will end at 17:30. Afterwards, we will go for dinner at 18:00. 

If you wish to attend, please fill out the sign-up form. Please complete the form by the end of Friday 20th November if you wish to join for dinner.

Schedule 

Title TBA [13:45-14:45] 

Łukasz Grabowski, Leipzig University 

Title TBA [13:00-14:00] 

Kaitlin Ragosta, EHU/UPV 

Coffee break [16:00-16:30] 

Title TBA [16:30-17:30] 

Laura Ciobanu, TU Berlin

April - July 2027, MATH+ Thematic Semester on Computational Algebra and Geometry, IMoS Building, Berlin

Short Timeline

April 04 – July 17 Advanced Courses of the Berlin Mathematical School start (FU/HU/TU) 

April 26–30 Kickoff Conference (TU)

June 14–18 Geometric and computational methods in enumerative algebra (TU) 

tba                Combinatorial Algebraic Geometry (HU) 

July 14 - July 16 OSCAR Summer School (TU)

 Past Events

 May 19, 2026 14:00 MA 366: Susan Hermiller (University of Nebraska)

Title: Subgroups of the group of dyadic piecewise linear homeomorphisms of the real line

Abstract: The group of dyadic orientation-preserving piecewise linear (PL) homeomorphisms of the unit interval is called Thompson's group F, and the question of which groups are - or cannot be - subgroups of F has yielded many interesting results. In this talk I'll discuss the question of what groups can or cannot be subgroups of Aut(F) (the automorphism group of F), and more particularly subgroups of an index 2 subgroup of Aut(F) that is isomorphic to a group of dyadic PL homeomorphisms of the real line. This is joint work with Conchita Martinez-Perez.

 May 12, 2026 14:00 MA 366: Marius Streiff (Heidelberg)

Title: Conjugation in T-RAAGs and graph products of groups

Abstract: Twisted right-angled Artin groups (T-RAAGs) and graph products of groups are two natural generalizations of right-angled Artin groups (RAAGs). The question arises, which nice geometric and algorithmic properties of right-angled Artin groups still hold for T-RAAGs and graph products of groups.

In this talk, I will present a geometric solution to the conjugacy problem in T-RAAGs and use it to compute the conjugator length function of T-RAAGs. If time permits, I will also give an outlook on the question how centralizers can be described in both graph products of groups and T-RAAGs using the quasi-median and median structure of their Cayley graphs.

May 7th 2026, EGGG University of Potsdam

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.

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.

 April 28, 2026 14:00 MA 366: Simon Andre (Paris)

Title: On the Tarski problem for hyperbolic groups in the presence of torsion

Abstract: Following his resolution of a famous problem of Tarski concerning elementary equivalence of non-abelian free groups, Sela gave a complete classification of torsion-free hyperbolic groups up to elementary equivalence. I will recall the necessary definitions, outline Sela's classification and explain some of the new difficulties encountered when elements of finite order are allowed, in particular the existence of pathological morphisms from surface groups to hyperbolic groups with torsion.

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.

 January 14, 2026 14:00 MA 366: Matthew Konefal (Loughborough, UK)

Title: Definability in Theories Based on Free Group and Free Monoid Equations

Abstract: Equations in the free group and free monoid have long been studied together. I’ll outline their history and properties, comparing facets of their two theories. Each can be studied by adapting techniques developed for the other. After demonstrating this, we’ll focus on definability in theories whose atoms are free group equations. Tools for establishing nondefinability in this setting are due to group theorists and model theorists, yet have the appearance of word-combinatorial lemmas. The tools - combined - yield characterisations of definability, the free monoid analogues of which seem more elusive. I'll end discussing length abstractions and length constraints, which link equations to weak arithmetics. Naturally, more definability questions result, including major open problems.

 November 12, 2025 14:00 MA 650: Kevin Li (FU Berlin)

Title: On higher coherence of RAAGs

Abstract: A group is called coherent if every finitely generated subgroup is

finitely presented. It is a classical result of Droms that a RAAG is coherent

if and only if the defining graph is chordal. I will explain its proof and our

attempt to generalise it to higher coherence. Ongoing joint work with Luis

Jorge Sánchez Saldaña.

 October 29, 2025 13:45 MA 366: Tamara Imanuel

Title: Das Konjugationsproblem in RAAGs: Theorie und Algorithmus

 October 8, 2025 10:45 MA 366: Alex Levine (University of East Anglia) 

Title: The Diophantine problem in Thompson's group F

Abstract: We discuss equations in groups, and when there exists an algorithm that can decide if a given system of equations admits a solution. We mention some recent work on Thompson's groups F and T. Based on joint work with Luna Elliott.