HSAGA Seminar

    In this talk, we will cover several aspects: Firstly, we will delve into the higher-dimensional Auslander-Reiten theory over an infinite-dimensional coalgebra; the finite-dimensional case by duality reduces to that of finite-dimensional algebra. We will introduce the n-transpose of a finitely n-copresented comodule and n-Auslander-Reiten translations, and then prove the n-Auslander-Reiten formula on n-cluster-tilting subcategories of comodule categories. Secondly, we will introduce the Gorenstein transpose via a minimal Gorenstein injective copresentation of a quasi-finite comodule, and explore the relations between the Gorenstein transpose of comodules and the transpose of the same comodule. As an application, we will construct the almost split sequences in terms of Gorenstein transpose. Finally, we will generalize the notion of the Auslander transpose of comodules to that of the transpose with respect to a semidualizing bicomodule T and showcase some nice homological properties of T-transpose. Additionally, we will provide a Foxby equivalence of comodule categories and investigate a characterization of T-reflexive comodules. This work is joint with Prof. Hailou Yao.

This talk aims to present the results obtained by Oussama Hamza, during his PhD studies, and his collaborators: Christian Maire, Jan Minac and Nguyen Duy Tan.

Their work precisely focuses on realisation of pro-p Galois groups over some fields with specific properties for a fixed prime p: especially filtrations and cohomology. Hamza was particularly interested on Number and Pythagorean fields.

This talk will mostly deal with the latest results obtained by Hamza and his collaborators on Formally real Pythagorean fields of finite type (RPF). For this purpose, they introduced a class of pro-2 groups, which is called $\Delta$-RAAGs, and studied some of their filtrations. Using previous work of Minac and Spira, Hamza and his collaborators showed that every pro-2 Absolute Galois group of a RPF is $\Delta$-RAAG. Conversely if a group is $\Delta$-RAAG and a pro-2 Absolute Galois group, then the underlying field is necessarily RPF. This gives us a new criterion to detect Absolute Galois groups.

Finally, the work of Hamza and his collaborators also proved that pro-2 Absolute Galois groups of RPF satisfy the Kernel unipotent conjecture; jointly introduced by Minac and Tan with the Massey vanishing conjecture, which attracted a lot of interest.

In 1996, A. Tamagawa discovered a surprising phenomenon: anabelian geometry also exists for curves over algebraically closed fields of characteristic p>0 (i.e., curves in positive characteristic can possibly be determined by their geometric fundamental groups without relying on Galois actions). However, after 28 years, only a few results have emerged in this field. In this talk, I want to explain the following insight of the speaker about fundamental groups of curves in positive characteristic: 

The (admissible or geometric log etale) fundamental groups of pointed stable curves over algebraically closed fields of characteristic p can be regarded as an analogue of local moduli spaces of the curves. 

This observation led to the speaker discovering some new kinds of anabelian phenomena of curves in characteristic p and to formulated numerious new conjectures. For example, the following highly non-trivial anbelian results of the speaker provide strong evidence supporting this insight:

• The homeomorphism conjecture holds for 1-dimensional moduli spaces (roughly speaking, this conjecture means that the moduli spaces of curves can be reconstructed group-theoretically as topological spaces).

• A new proof of Mochizuki’s famous result concerning (Isom-version) Grothendieck’s anabelian conjecture for curves over sub-p-adic fields without using Faltings’ p-adic Hodge theory.

• The group-theoretical specialization conjecture holds (roughly speaking, this conjecture means that the topological and group-theoretical degeneration of curves can be completely determined by open continuous homomorphisms of dmissible fundamental groups).

    In this talk, I will discuss arithmetic purity of strong approximation: motivations and related results. Finally, for complete toric varieties, I will briefly explain my work on this topic.

    For any reductive group we find a stacky interpretation of the stability condition for principal bundles over a curve: it is the unique maximal open locus that admits a schematic moduli space. Some applications and further progress will be discussed. This is a joint work with Dario Weissmann and Andres Fernandez Herrero.

    I shall start from a classical result on Brauer-Manin obstruction of a product of two varieties, by introducing Yang Cao's formulas of Kunneth type. Then we will generalize it to a class of algebraic stacks.