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Spring Semester 2026

Date / Time Speaker Title Location
18 February 2026
15:30-16:30
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Geometry Seminar

Title kein Seminar
Speaker, Affiliation
Date, Time 18 February 2026, 15:30-16:30
Location HG G 43
kein Seminar
HG G 43
25 February 2026
15:30-16:30
Details

Geometry Seminar

Title kein Seminar
Speaker, Affiliation
Date, Time 25 February 2026, 15:30-16:30
Location HG G 43
kein Seminar
HG G 43
4 March 2026
15:30-16:30 		Thomas Nikolaus
Münster
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Geometry Seminar

Title Singular chains as Coalgebras
Speaker, Affiliation Thomas Nikolaus, Münster
Date, Time 4 March 2026, 15:30-16:30
Location HG G 43
Abstract A very old and natural question in topology is how much information about a space X is contained in its singular homology H_*(X). It is well known that homology by itself is not a complete invariant: different spaces can have exactly the same homology groups without being homotopy equivalent. The situation becomes much more interesting once additional structure is taken into account. Instead of passing immediately to homology, one can look at the full chain complex C_*(X) together with the cross product, which is closely related to the cup product on cochains. This extra structure encodes subtle interactions between chains that are lost on homology, and it is responsible for familiar phenomena such as Massey products and Steenrod operations. In this talk, we discuss the result that singular cochains, when viewed as an E_infinity coalgebra, actually form a complete invariant of a space: the homotopy type of X can be recovered from this structure in a functorial way. This generalizes classical rational homotopy theory as well as deep results of Mandell, Yuan and Bachmann--Hahn. More generally, we explain how the result is based on a classification of "perfect" E-infinity coalgebras. All of this is joint work with F. Riedel.
Singular chains as Coalgebrasread_more
HG G 43
11 March 2026
15:30-16:30 		Marc Abboud
Université de Neuchâtel
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Geometry Seminar

Title A uniform bound on common periodic points in families of regular plane polynomial automorphisms
Speaker, Affiliation Marc Abboud, Université de Neuchâtel
Date, Time 11 March 2026, 15:30-16:30
Location HG G 43
Abstract A regular plane polynomial automorphism is a subclass of polynomial diffeomorphism f: C^2 - > C^2 of the complex affine plane with positive topological entropy. Every polynomial diffeomorphism of positive topological entropy is conjugated to a regular one. Dujardin and Favre showed that two such transformations cannot share infinitely many periodic points unless they share a common iterate. We show that this results holds uniformly in a family: Take two 1-parameter families of regular plane polynomial automorphisms (f_t) and (g_t), then there exists a constant D >0 such that for every parameter t except for a finite number of them f_t and g_t share at most D periodic points. This generalises a result of Mavraki and Schmidt who showed it for endomorphisms of the projective line. I will give a brief overview of the history of this problem and explain key tools of the proof which is based on the theory of adelic divisors and arithmetic equidistribution. This is joint work with Yugang Zhang.
A uniform bound on common periodic points in families of regular plane polynomial automorphismsread_more
HG G 43
18 March 2026
15:30-16:30 		Kathryn Hess Bellwald
EPFL
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Geometry Seminar

Title Hochschild homology: the universal shadow
Speaker, Affiliation Kathryn Hess Bellwald, EPFL
Date, Time 18 March 2026, 15:30-16:30
Location HG G 43
Abstract Hochschild homology has proved to be an important invariant in algebra and homotopy theory, in particular due to its relevance in algebraic K-theory and fixed point theory, leading to the development of numerous variants of the original construction. Ponto's theory of shadows provides a bicategorical axiomatization of Hochschild homology-type invariants, which captures the essential common properties of all known variants of Hochschild homology, such as Morita invariance. In recent work, Nima Rasekh and I clarified the relationship between shadows and Hochschild homology. After extending the notion of Hochschild homology to bicategories in a natural manner, we proved the existence of a universal shadow on any bicategory B, taking values in the Hochschild homology of B, through which all other shadows on B factor. Shadows are thus co-represented by a bicategorical version of Hochschild homology. Using the universal shadow on the free adjunction bicategory, we established a universal Morita invariance theorem, of which all known cases are immediate corollaries. In this talk I will give an overview of my work with Rasekh and provide relevant examples of shadows, including the free loop space construction, then discuss potential generalization and extensions of our results.
Hochschild homology: the universal shadowread_more
HG G 43
25 March 2026
15:30-16:30 		Cameron Rudd
University of Oxford
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Geometry Seminar

Title PD3 + (T)
Speaker, Affiliation Cameron Rudd, University of Oxford
Date, Time 25 March 2026, 15:30-16:30
Location HG G 43
Abstract I will discuss how the strong expansion properties of residually finite groups with Kazhdan's property (T) are incompatible with three dimensional Poincaré duality in the sense of Wall.
PD3 + (T)read_more
HG G 43
1 April 2026
15:30-16:30 		Alexander Lytchak
Karlsruhe Institute of Technology
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Geometry Seminar

Title Metric-measure boundary and geodesic flow on singular spaces
Speaker, Affiliation Alexander Lytchak, Karlsruhe Institute of Technology
Date, Time 1 April 2026, 15:30-16:30
Location HG G 43
Abstract In the talk I will disuss the existence of the geodesic flow and the invariance of the Liouville measure in non-smooth settings. A central role will play the so-called metric-measure boundary, an object from the geometric measure theory, which detects the boundary of a Riemannian manifold in the smooth setting and controls the average non-flatness of the space in more general situations. The talk will be based on a joint work with Vitaly Kapovitch and Anton Petrunin and an ongoing project with Daniele Semola and Stephan Stadler.
Metric-measure boundary and geodesic flow on singular spacesread_more
HG G 43
8 April 2026
15:30-16:30
Details

Geometry Seminar

Title Easter break
Speaker, Affiliation
Date, Time 8 April 2026, 15:30-16:30
Location HG G 43
Easter break
HG G 43
15 April 2026
15:30-16:30 		Marc Kegel
Universidad de Sevilla
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Geometry Seminar

Title The search for exotic knot traces
Speaker, Affiliation Marc Kegel, Universidad de Sevilla
Date, Time 15 April 2026, 15:30-16:30
Location HG G 43
Abstract Every knot leaves a trace in the 4-dimensional world. The trace of a knot is the smooth 4-manifold obtained by attaching a 2-handle to the 4-ball along a knot in the 3-sphere. We will introduce the relevant notions and present a strategy to disprove the smooth 4-dimensional Poincaré conjecture by finding knot traces with certain exotic properties. In the second part of the talk, we will discuss different methods to search for such exotic knot traces. This talk will mainly be based on joint work with Jonathan Spreer.
The search for exotic knot tracesread_more
HG G 43
22 April 2026
15:30-16:30 		Ara Basmajian
City University of New York
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Geometry Seminar

Title From Collars on Riemann surfaces to Tubes in Complex Ball quotients
Speaker, Affiliation Ara Basmajian, City University of New York
Date, Time 22 April 2026, 15:30-16:30
Location HG G 43
Abstract The celebrated Keen collar lemma guarantees that a simple closed geodesic on a hyperbolic Riemann surface has a collar (tubular neighborhood) whose width only depends on its length. Viewing a Riemann surface as the quotient of the unit ball in the complex plane, a natural generalization is to ball quotients in higher dimensions where the Poincaré metric is replaced by the Bergman metric (also known as the complex hyperbolic metric). Such ball quotients are called complex hyperbolic manifolds. The focus of this talk will be on embedded complex geodesics in complex hyperbolic 2-manifolds; a complex geodesic has complex codimension one in the quotient complex 2-manifold. We prove a tubular neighborhood theorem for such a complex geodesic where the width of the tube depends only on the Euler characteristic of the embedded complex geodesic. We also derive an explicit estimate for this width. After giving a short history of the collar lemma generalizations and discussing the basics of complex hyperbolic geometry, we will discuss the ideas leading to the proof of this tubular neighborhood theorem. This is joint work with Youngju Kim.
From Collars on Riemann surfaces to Tubes in Complex Ball quotientsread_more
HG G 43
13 May 2026
15:30-16:30 		Daniela Paiva Peñuela
Universität Basel
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Geometry Seminar

Title Automorphisms of quartic surfaces and Cremona transformations
Speaker, Affiliation Daniela Paiva Peñuela, Universität Basel
Date, Time 13 May 2026, 15:30-16:30
Location HG G 43
Abstract Gizatullin's problem consists of determining which automorphisms of a smooth quartic surface S in P^3 arise from birational transformations of P^3. This problem fits into a more general question: to characterize which automorphisms of a smooth hypersurface in projective space are restrictions of projective transformations. This question was completely solved by Matsumura and Monsky (1964), and Chang (1978), except for two cases: that of an elliptic curve in P^2 and that of a quartic surface in P^3. For elliptic curves, it is known that although their automorphisms do not come from projective transformations of P^2, they do arise from birational transformations. Therefore, the only open case corresponds to quartic surfaces in P^3, which we refer to as Gizatullin's problem. In this seminar, I will provide a general background on the theory of K3 surfaces and the birational geometry of P^3, and explain how the interplay between these areas can be exploited to address the problem. In particular, I will present a solution to Gizatullin's problem in certain specific cases. The results I will present are part of a series of joint works with Carolina Araujo, Ana Quedo, and Sokratis Zikas.
Automorphisms of quartic surfaces and Cremona transformationsread_more
HG G 43
20 May 2026
15:30-16:30 		Alonso Beaumont
Rennes
Details

Geometry Seminar

Title The Margulis-Soifer property for the planar Cremona group
Speaker, Affiliation Alonso Beaumont, Rennes
Date, Time 20 May 2026, 15:30-16:30
Location HG G 43
Abstract In 1981, Margulis and Soifer established a remarkable property of finitely generated linear groups: such a group contains uncountably many maximal subgroups if and only if it is not virtually solvable. In particular, this yields the existence of maximal subgroups of infinite index in SLn(Z). We will show how their methods can be adapted to establish an analogous result for groups of birational transformations of the plane. This is based on joint work with Serge Cantat, Julie Déserti and Seung Uk Jang.
The Margulis-Soifer property for the planar Cremona groupread_more
HG G 43
27 May 2026
15:30-16:30 		Francesco Lin
Columbia University
Details

Geometry Seminar

Title Coexact 1-form spectral gaps and three-dimensional topology
Speaker, Affiliation Francesco Lin, Columbia University
Date, Time 27 May 2026, 15:30-16:30
Location HG G 43
Abstract The spectral gap of the Hodge Laplacian on coexact 1-forms is a fundamental quantity associated to a Riemannian manifold which has attracted quite a lot of attention in recent years. I will begin by discussing the role it plays in topological problems about three-manifolds (such as the existence on taut foliations on rational homology spheres) via its relation with Floer theoretic invariants. Motivated by this, I will then focus on the case of hyperbolic manifolds, and describe techniques to determine explicitly such spectral gaps in concrete examples (including certain infinite families). This is joint work with M. Lipnowski.
Coexact 1-form spectral gaps and three-dimensional topologyread_more
HG G 43

Notes: the highlighted event marks the next occurring event.

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

Archive: SS 26 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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