Séminaire de Mathématique

Cohomological Descent for Faltings' p-adic Hodge Theory and Applications

by Tongmu He (IHES & Université Paris-Saclay)

Amphithéâtre Léon Motchane (IHES)

Amphithéâtre Léon Motchane


Le Bois Marie 35, route de Chartres 91440 Bures-sur-Yvette

Séminaire de géométrie arithmétique

Faltings' approach in p-adic Hodge theory can be schematically divided into two main steps: firstly, a local reduction of the computation of the p-adic étale cohomology of a smooth variety over a p-adic local field to a Galois cohomology computation and then, the establishment of a link between the latter and differential forms. These relations are organized through Faltings ringed topos whose definition relies on the choice of an integral model of the variety, and whose good properties depend on the (logarithmic) smoothness of this model. Scholze's generalization for rigid analytic varieties has the advantage of depending only on the variety (i.e. the generic fibre). Inspired by Deligne's approach to classical Hodge theory for singular varieties, we establish a cohomological descent result for the structural sheaf of Faltings topos, which makes it possible to extend Faltings' approach to any integral model, i.e. without any smoothness assumption. An essential ingredient of our proof is a descent result of perfectoid algebras in the arc-topology due to Bhatt and Scholze. 

As an application of our cohomological descent, using a variant of de Jong's alteration theorem for morphisms of schemes due to Gabber-Illusie-Temkin, we generalize Faltings' main p-adic comparison theorem to any proper and finitely presented morphism of coherent schemes over an absolute integral closure of Z_p (without any assumption of smoothness) for torsion étale sheaves (not necessarily finite locally constant). As a second application, we deduce a local version of the relative Hodge-Tate filtration from the global version constructed by Abbes-Gros.


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Organized by

Christophe Soulé et Jeanine Van Order