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Autumn Semester 2025

Date / Time Speaker Title Location
17 September 2025
15:30-16:30
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Geometry Seminar

Title kein Seminar
Speaker, Affiliation
Date, Time 17 September 2025, 15:30-16:30
Location HG G 43
kein Seminar
HG G 43
24 September 2025
15:30-16:30 		Thomas Haettel
University of Montpellier
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Geometry Seminar

Title Nonpositively curved simplicial complexes
Speaker, Affiliation Thomas Haettel, University of Montpellier
Date, Time 24 September 2025, 15:30-16:30
Location HG G 43
Abstract We will present injective metric spaces, which are metric spaces where any family of pairwise intersecting balls has a non-empty global intersection. We will explain how they can be used to develop a rich theory of nonpositively curved simplicial complexes, somewhat parallel to the theory of CAT(0) cube complexes. We will show that they arise naturally for numerous spaces and groups: hyperbolic groups, buildings, braid groups, Artin groups, Garside groups, some arc complexes on surfaces...
Nonpositively curved simplicial complexesread_more
HG G 43
* 25 September 2025
13:00-14:00 		Damian Osajda
Copenhagen / Wroclaw
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Geometry Seminar

Title Drilling Hyperbolic Groups
Speaker, Affiliation Damian Osajda, Copenhagen / Wroclaw
Date, Time 25 September 2025, 13:00-14:00
Location HG G 19.2
Abstract Drilling a closed hyperbolic 3-manifold along an embedded geodesic is a crucial technique in low-dimensional topology, transforming the fundamental group of the manifold into a relatively hyperbolic group. In this talk, we extend this concept by proving that, under appropriate conditions, a similar "drilling" operation can be applied to a (Gromov) hyperbolic group with the 2-sphere boundary. Our primary motivations and applications revolve around the Cannon Conjecture, which states that if the Gromov boundary of a hyperbolic group is homeomorphic to the 2-sphere, then the group is virtually (i.e., up to a finite-index subgroup) the fundamental group of a closed 3-manifold of constant negative curvature. We also explore the relatively hyperbolic counterpart—the Toral Relative Cannon Conjecture. Using drilling, we show that if the Toral Relative Cannon Conjecture holds, then the Cannon Conjecture is valid for all residually finite hyperbolic groups. The Toral Relative Cannon Conjecture appears more accessible, owing to the presence of additional structure—abelian parabolic subgroups. This is joint work with Daniel Groves, Peter Haïssinsky, Jason Manning, Alessandro Sisto, and Genevieve Walsh.
Drilling Hyperbolic Groupsread_more
HG G 19.2
1 October 2025
15:30-16:30 		Dr. Damaris Meier
ETH Zurich, Switzerland
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Geometry Seminar

Title Uniformization of metric surfaces
Speaker, Affiliation Dr. Damaris Meier, ETH Zurich, Switzerland
Date, Time 1 October 2025, 15:30-16:30
Location HG G 43
Abstract The uniformization problem for metric surfaces asks under which condition a metric space X, homeomorphic to a model surface M, admits a parametrization u: M → X with good geometric and analytic properties. In this talk, we focus on the case where X has locally finite Hausdorff 2-measure. After revisiting the breakthrough results of Bonk-Kleiner and Rajala, we will demonstrate that no additional assumptions are necessary for the existence of a "good" parametrization. If X is locally geodesic, such parametrizations can be constructed by exploiting existence and regularity properties of energy-minimizing Sobolev mappings.
Uniformization of metric surfacesread_more
HG G 43
8 October 2025
15:30-16:30 		David Cimasoni
University of Geneva
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Geometry Seminar

Title Signatures of knots and links
Speaker, Affiliation David Cimasoni, University of Geneva
Date, Time 8 October 2025, 15:30-16:30
Location HG G 43
Abstract The signature is one of the most versatile invariants in knot theory. For example, it can be used to detect the chirality of a knot, and yields lower bounds on the unknotting number, the Seifert genus, as well as the four-genus of a knot. This invariant also extends nicely to links, offering new insights such as bounds on the splitting number. The aim of this talk is to give a gentle introduction to this powerful invariant, tracing its development from Trotter’s original definition in 1962 to the most recent advances.
Signatures of knots and linksread_more
HG G 43
15 October 2025
15:30-16:30 		Jérémy Blanc
Université de Neuchâtel
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Geometry Seminar

Title Topological simplicity of the group of automorphisms of the affine plane
Speaker, Affiliation Jérémy Blanc, Université de Neuchâtel
Date, Time 15 October 2025, 15:30-16:30
Location HG G 43
Abstract The group AUT(A^n) of polynomial automorphisms of the space is not a simple group, as it contains the closed subgroup SAUT(A^n) of Jacobian 1. The group SAUT(A^2) is known to be abstractly not simple by a result of Danilov of 1974. The group SAUT(A^n) was however claimed to be topologically simple (no non-trivial closed normal subgroups) by Shafarevich in 1981. We will here prove this claim in dimension n=2 and give some ideas about the situation in higher dimension. This is over an infinite field, the case of finite fields being dramatically different.
Topological simplicity of the group of automorphisms of the affine planeread_more
HG G 43
22 October 2025
15:30-16:30
Details

Geometry Seminar

Title kein Seminar
Speaker, Affiliation
Date, Time 22 October 2025, 15:30-16:30
Location HG G 43
kein Seminar
HG G 43
29 October 2025
15:30-16:30 		Alan Reid
Rice
Details

Geometry Seminar

Title Profinite rigidity, low-dimensional topology and Grothendieck Pairs
Speaker, Affiliation Alan Reid, Rice
Date, Time 29 October 2025, 15:30-16:30
Location HG G 43
Abstract The set of finite quotients of a finitely generated group is neatly captured by its profinite completion. A finitely generated (residually finite) group G is called profinitely rigid if whenever another finitely generated (residually finite) group H has isomorphic profinite completion, then H is isomorphic to G. This talk will discuss recent progress on groups that are (are not) profinitely rigid, and in particular describe a construction (Grothendieck Pairs) that produce a finitely presented group that is rigid amongst finitely presented groups, but not amongst finitely generated ones. An emphasis will be placed on the central role that 3-manifold groups play.
Profinite rigidity, low-dimensional topology and Grothendieck Pairsread_more
HG G 43
5 November 2025
15:30-16:30
Details

Geometry Seminar

Title kein Seminar
Speaker, Affiliation
Date, Time 5 November 2025, 15:30-16:30
Location HG G 43
kein Seminar
HG G 43
12 November 2025
15:30-16:30
Details

Geometry Seminar

Title kein Seminar
Speaker, Affiliation
Date, Time 12 November 2025, 15:30-16:30
Location HG G 43
kein Seminar
HG G 43
19 November 2025
15:30-16:30 		Jean Raimbault
Marseille
Details

Geometry Seminar

Title Arithmetic link complements
Speaker, Affiliation Jean Raimbault, Marseille
Date, Time 19 November 2025, 15:30-16:30
Location HG G 43
Abstract Two classical constructions of cusped hyperbolic 3–manifolds of finite volume are (some) link complements in the sphere, and quotients of hyperbolic space by congruence subgroups of Bianchi groups such as PSL2(Z[i]). A conjecture of Baker and Reid posits that only finitely many manifolds occur as both. I will discuss this conjecture in relation with well-known and conjectured properties of arithmetic groups, and present a proof of the conjecture obtained in joint work with S. Kionke.
Arithmetic link complementsread_more
HG G 43
26 November 2025
15:30-16:30 		Fernando Camacho Cadena
Strasbourg
Details

Geometry Seminar

Title Hamiltonian flows on (higher rank) Teichmüller spaces and self-intersecting curves
Speaker, Affiliation Fernando Camacho Cadena, Strasbourg
Date, Time 26 November 2025, 15:30-16:30
Location HG G 43
Abstract A hyperbolic structure on a surface S can be deformed by cutting along a curve c without self-intersections, twisting, and re-gluing. Such a deformation can also be seen as the Hamiltonian flow of the length function of c on the Teichmüller space of S, equipped with the Weil—Petersson symplectic form. In this talk, I will discuss Hamiltonian flows associated to length functions of self-intersecting curves, and their generalizations to higher rank Teichmüller spaces. This is partially joint work with J. Farre and A. Wienhard.
Hamiltonian flows on (higher rank) Teichmüller spaces and self-intersecting curvesread_more
HG G 43
3 December 2025
15:30-16:30 		Stéphane Lamy
Toulouse
Details

Geometry Seminar

Title Polynomial automorphisms and negative curvature
Speaker, Affiliation Stéphane Lamy, Toulouse
Date, Time 3 December 2025, 15:30-16:30
Location HG G 43
Abstract Polynomial or birational transformations of ℂn form huge groups as soon as n > 1, and the following basic questions are open in general:
  • Do they admit normal subgroups (apart from the obvious subgroup of Jacobian 1 automorphisms in the polynomial case)?
  • Do they satisfy a Tits alternative, as in the linear case?
  • Is any finite subgroup conjugate to a subgroup of GL(n)?
After explaining the context, I will discuss some particular cases in dimensions 3 and 4 where we can answer these questions, using some actions on some metric spaces with negative curvature. (based on several joint works with P. Przytycki)
Polynomial automorphisms and negative curvatureread_more
HG G 43
10 December 2025
15:30-16:30 		Antoine Pinardin
Universität Basel
Details

Geometry Seminar

Title Finite simple subgroups of the real Cremona group of rank three
Speaker, Affiliation Antoine Pinardin, Universität Basel
Date, Time 10 December 2025, 15:30-16:30
Location HG G 43
Abstract Very little is known about the classification of finite subgroups of Cremona in dimension three. It is natural to start with the case of simple groups, and this step was achieved by Prokhorov in 2009 over the field of complex numbers. In the work I will present, we show that the only non-cyclic finite simple subgroups of the real Cremona group of rank three are A5 and A6. This is a joint project with I. Cheltsov and Y. Prokhorov.
Finite simple subgroups of the real Cremona group of rank threeread_more
HG G 43
17 December 2025
15:30-16:30 		Peter Feller
Université de Neuchâtel
Details

Geometry Seminar

Title Monodromies of non-fibered 3-manifolds
Speaker, Affiliation Peter Feller, Université de Neuchâtel
Date, Time 17 December 2025, 15:30-16:30
Location HG G 43
Abstract While compact oriented connected manifolds of dimension 1 (the circle and the interval) and 2 (genus g surfaces with r boundary components) are readily classified, the study of 3-manifolds is an active research area with many competing perspectives, including the celebrated geometrization program initiated by Thurston. Among 3-manifolds, the fibered ones—those with a regular map to the circle S1—are arguably the simplest to study, as their properties can be fully described in terms of their monodromy: the gluing self-map of the fiber (a surface) of a chosen regular map to S1. For example, irreducibility and atoroidality (the topological properties of not containing interesting spheres or tori) and hyperbolicity (the geometric property of featuring a metric with sectional curvature –1) are readily discerned from the properties of the monodromy. Famously, Thurston's hyperbolization criterion says a fibered 3-manifold is hyperbolic if and only if the monodromy is neither reducible nor periodic.
Based on joint work with Lewark–Orbegozo Rodriguez and Orbegozo Rodriguez, we describe how to associate a monodromy to any irreducible surface (a so-called Haken surface) Σ in a 3-manifold that need not be a fiber of a regular map. Our setup is chosen to allow for an analog of Thurston's hyperbolization criterion. We illustrate our approach by providing new results concerning irreducibility, atoroidality and hyperbolicity for a particularly visualizable class of 3-manifolds: the exteriors of knots in the 3-sphere. In terms of technology, we use classical decomposition theory and the language of product discs and annuli as pioneered by Gabai to define a notion of monodromy that takes the form of a partially defined self-map of the arc and curve graph of Σ.
Monodromies of non-fibered 3-manifoldsread_more
HG G 43

Notes: the highlighted event marks the next occurring event, red marked events are important and events marked with an asterisk (*) indicate that the time and/or location are different from the usual time and/or location.

Organisers: Manfred Einsiedler, Urs Lang, Lukas Lewark, Christian Urech

Archive: AS 25 SS 25 AS 24 SS 24 AS 23 SS 23 AS 22 SS 22 AS 21 AS 20 SS 20 AS 19 SS 19 AS 18 SS 18 AS 17 SS 17 AS 16 SS 16 AS 15 SS 15 AS 14 SS 14 AS 13 SS 13 AS 12 SS 12 AS 11 SS 11 AS 10 SS 10 AS 09 SS 09

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