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Néron blowups and low-degree cohomological applications


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 $G$.
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Dates and versions

hal-02490010 , version 1 (24-02-2020)
hal-02490010 , version 2 (13-03-2020)



Arnaud Mayeux, Timo Richarz, Matthieu Romagny. Néron blowups and low-degree cohomological applications. 2020. ⟨hal-02490010v2⟩
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