N\'eron blowups and low-degree cohomological applications

Arnaud Mayeux (1, 2) , Timo Richarz (3) , Matthieu Romagny (4)

1 Beijing International Center for Mathematical Research
2 Peking University [Beijing]
3 TU Darmstadt - Technische Universität Darmstadt - Technical University of Darmstadt
4 IRMAR - Institut de Recherche Mathématique de Rennes

We define dilatations of general schemes and study their basic properties. Dilatations of group schemes are -- in favorable cases -- again group schemes, called N\'eron blowups. We give two applications to their cohomology in degree zero (integral points) and degree one (torsors): we prove a canonical Moy-Prasad isomorphism that identifies the graded pieces in the congruent filtration of $G$ with the graded pieces in its Lie algebra $\mathfrak g$, and we show that many level structures on moduli stacks of $G$-bundles are encoded in torsors under N\'eron blowups of $G$.

Domaines: Géométrie algébrique [math.AG]

Arnaud Mayeux, Timo Richarz, Matthieu Romagny. N\'eron blowups and low-degree cohomological applications. 2020. ⟨hal-02490010v1⟩