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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 Talks
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.
May 8, 2026 14:15 MA 001, MATH+ Friday Colloquium: Susan Hermiller (University of Nebraska)
Title: A tale of three unknotting conjectures
Abstract: A knot is a circle embedded in 3-space; two knots are considered to be the same if we can deform one to the other, without breaking the circle or letting it pass through itself. Unknotting number is a fundamental measure of how complicated a knot is, measuring how far it is from the unknot via crossing changes. Unknotting number is a challenging invariant to compute; a vast array of tools have been applied to its calculation, and many conjectures have grown up around it.
In this talk, Hermiller will discuss three conjectures, each aimed at simplifying the task of computing unknotting numbers. She will describe how her resolution of one of these conjectures several years ago, in joint work with Mark Brittenham, led them recently to resolve another – the (non)additivity of unknotting number under connected sum.
Susan Hermiller is a Willa Cather Professor of Mathematics at the University of Nebraska. Her research focuses on the interplay between computational, combinatorial, and geometric properties in in group theory and low dimensional topology. After earning her Ph.D. at Cornell under the direction of Prof. Ken Brown, sheheld postdoctoral positions at the Mathematical Sciences Research Institute (now SLMath) and the University of Melbourne, and a faculty position at New Mexico State University, before moving to Nebraska. She is a Fellow of the American Mathematical Society.
May 12, 2026 14:00 MA 366: Marius Streiff (Heidelberg)
Title: Conjugation in T-RAAGs and graph products of groups
The 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 12, 2026 15:00 MA 366: Islam Foniqi (Berlin)
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Past Talks
April 29, 2026 16:30 EN 058: Laura Ciobanu (Berlin). In the Diskrete Mathematik/Geometrie Seminar
Title: Counting in groups
Abstract: In this talk I will give an overview on growth (of elements, conjugacy classes etc) in groups and discuss both the asymptotics and the formal series associated to the objects we are counting. I will highlight how the rationality (or lack thereof) of these series is connected to both the algebraic and the geometric nature of the groups.
I will then focus on virtually abelian groups, that is, groups that have an abelian subgroup of finite index. The rationality of various growth series in virtually abelian groups is particularly striking, as it holds for any (weighted) generating set. The tools to prove rationality come from discrete geometry, combinatorics, theoretical computer science and linear algebra.
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.
April 21, 2026 14:00 MA 366: Corentin Bodart (Oxford)
Title: Subgroup membership and formal languages
Abstract: The intersection of Geometric Group Theory and Formal Language Theory has been fruitful in the last 50 years. I'll start by recalling some of the key results in the area, such as the Muller-Schupp theorem on groups with context-free Word Problem and Lehnert's conjecture on groups with co-context-free Word Problem. In joint work with André Carvalho and Carl-Fredrik Nyberg-Brodda, we have been looking at similar languages related to another decision problem in groups: Subgroup Membership. I'll explain the toolbox we have been developing, apply it to examples and non-examples, and highlight some open problems along the way.
January 21, 2026 14:00 MA 366: Jan Moritz Petschick (Uni Bielefeld)
Title: Maximal subgroups in branch groups
Abstract: Branch groups are groups whose subgroup structure resembles a rooted tree. Among many other intriguing properties, they contain examples of groups with unusual restrictions on their maximal subgroups. In this talk, I will provide an account of what is known about maximal subgroups of branch groups and present some recent results.
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.
December 10, 2025 14:30 ER 164: Leon Pernak (Uni Saarland)
Title: Quadratic equations in wreath products of abelian groups
Abstract: One of the strongest results that one can hope for when studying decidability questions in groups is the decidability of equations - is there an algorithm that, if we feed it a group equation, tells us if the equation has or does not have a solution in a given group? I will discuss this problem in the setting of wreath products of abelian groups. In particular, I will explain how to prove that the problem is decidable for quadratic equations, using techniques and intuitions inspired by commutative algebra. This is joint work with Ruiwen Dong and Jan-Philipp Wächter.
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 14:30 MA 650: Mireille Soergel (TUB)
Title: Dual Artin groups
Abstract: In 1998 when considering a new approach to the word and conjugacy problem in Braid groups, Birman-Ko-Lee introduce a new presentation for the braid group. This is now referred to as the dual presentation. Such dual presentations can be defined for all Artin groups. I will introduce standard and dual Artin groups and talk about some open conjectures.
October 29, 2025 13:45 MA 366: Tamara Imanuel
Title: Das Konjugationsproblem in RAAGs: Theorie und Algorithmus
October 22, 2025 14:30 MA 650: Sobhi Massalha (TUB)
Title: Sentences over Random Groups
Abstract: Around 2013, a natural conjecture by Julia Knight anticipated
that given a first order sentence, its truth value over a random group in
the few relators model, is equivalent to its truth value over non-abelian
free groups.
Our work extends the framework of this conjecture to random groups in the
Gromov Density Model, at optimal density d<1/2.
We prove the conjecture for sentences that belong to the Boolean algebra
of universal sentences, as well as for sentences of minimal rank with
arbitrary number of quantifiers, for random groups of density d<1/2.
In this talk, we will present our results and, as time permits, outline
the key strategies and ideas involved in the proofs.
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.
October 8, 2025 10:00 MA 366: Nico Waldau (TU Berlin)
Title: Tits-Alternative für hyperbolische Gruppen
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