The Bielefeld Algebraic and Arithmetic Geometry Seminar SS2026

Welcome to the website of the Bielefeld Algebraic and Arithmetic Geometry Seminar! For more information, please contact one of the organizers: Claudia Alfes, Ana Botero, Lennart Gehrmann, Fabian Hebestreit, Eike Lau, Michael Spiess, Charles Vial, and Thomas Zink.

Talks

Wednesday 29 April, 2026
14:15	X-E0-209	Zekun Ji (Amsterdam) De Rham-Betti Structures and Groups for Abelian Varieties [Abstract] In this talk, we discuss de Rham-Betti structures and groups for abelian varieties over Qbar. This is a framework introduced by Yves André to study the Grothendieck period conjecture, and further developed by Bost-Charles and Kreutz-Shen-Vial etc. I show that the de Rham-Betti group of any abelian variety contains the subgroup of homotheties. And I will explain the techniques involved to explicitly compute the de Rham-Betti groups in several special cases of abelian varieties. The results are based on my PhD thesis.
15:45	TBA	TBA TBA [Abstract] TBA
Wednesday 13 May, 2026
14:15	TBA	TBA TBA [Abstract] TBA
15:45	TBA	TBA TBA [Abstract] TBA
Wednesday 27 May, 2026
14:15	TBA	TBA TBA [Abstract] TBA
15:45	V2-205	TBA TBA [Abstract] TBA
Wednesday 10 June, 2026
14:15	TBA	Carolina Tamborini (Duisburg-Essen) Arakelov inequalities and characterization of totally geodesic ball quotients [Abstract] I will report on joint work with Matteo Costantini and Daniel Greb providing an intrinsic numerical characterization of totally geodesic ball quotients inside the moduli space of principally polarized abelian varieties over the complex numbers, obtained in terms of an Arakelov (in)equality associated with the underlying variation of Hodge structure. Our result extends work of Möller, Viehweg, and Zuo by removing some positivity conditions imposed in their statement. Our approach involves first showing that the period map associated with a family of Abelian varieties factors through certain MMP operations, and then generalizing the results of Möller, Viehweg, and Zuo to a singular setting.
15:45	TBA	Ellen Eischen (Princeton U) Algebraicity of Spin L-functions for GSp_6 [Abstract] I will discuss recent results for algebraicity of critical values of Spin L-functions for GSp_6. I will also discuss ongoing work toward the construction of p-adic L-functions interpolating these values. I will explain how this work fits into the context of earlier developments, while also indicating where new technical challenges arise. This is joint work with Giovanni Rosso and Shrenik Shah. All who are curious about this topic are welcome at this talk, even without prior experience with Spin L-functions.
Wednesday 08 July, 2026
14:15	TBA	TBA TBA [Abstract] TBA
15:45	TBA	TBA TBA [Abstract] TBA
Friday 22 July, 2026
14:15	TBA	Damian Rössler (Oxford U) On a conjecture of Esnault and Langer [Abstract] Let A be an abelian variety over a field K, which is of positive characteristic and which is finitely generated over an algebraically closed field. One can associate with A an infinite sequence A <-- A^(p) <-- A^(p^2) <-- A^(p^3) <-- … of abelian varieties, where A^(p^k) is the k-th Frobenius twist of A over K, and the arrows <-- are Verschiebung morphisms. Esnault and Langer conjectured in 2013 that an element of A(K), which has K-rational preimages by all the compositions of arrows in this sequence (ie an “indefinitely Verschiebung divisible point”), must be a torsion point of order prime to the characteristic, once the largest isotrivial part of A is quotiented out. We will give an outline of the proof of this statement that was given by the speaker in 2020. The proof is based on the theory of semistable sheaves on curves, and its relation to the classification of finite flat group schemes.
15:45	TBA	Thomas Agugliaro (Strasbourg) Hodge standard conjecture for powers [Abstract] The Hodge standard conjecture predicts positivity of intersection forms on algebraic cycles. It was formulated by Grothendieck in the Sixties, motivated by an intersection theoretic proof of the Weil bound for curves over finite fields. Only recently some progress has been made, based on p-adic Hodge theory. As most conjectures on algebraic cycles, it behaves badly under powers. In this talk, we will investigate this question and prove the conjecture for powers of abelian varieties of dimension 3.

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Seminars

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The Bielefeld Algebraic and Arithmetic Geometry Seminar SS2026

Talks

Wednesday

29 April, 2026

Wednesday

13 May, 2026

Wednesday

27 May, 2026

Wednesday

10 June, 2026

Wednesday

08 July, 2026

Friday

22 July, 2026