On the crystalline cohomology of Deligne-Lusztig varieties

TL;DR

This paper constructs a logarithmic F-crystal on a smooth compactification to relate crystalline and rigid cohomology of certain Galois covers, with applications to Deligne-Lusztig varieties over finite fields.

Contribution

It introduces a natural $W[F]$-lattice in the crystalline cohomology of Galois covers, providing a new tool for understanding the cohomology of Deligne-Lusztig varieties.

Findings

Constructed a logarithmic F-crystal on compactifications.

Established a G-equivariant $W[F]$-lattice in cohomology.

Expressed reduction modulo p in terms of equivariant Hodge cohomology.

Abstract

Let $X\to Y^0$ be an abelian prime-to-$p$ Galois covering of smooth schemes over a perfect field $k$ of characteristic $p>0$. Let $Y$ be a smooth compactification of $Y^0$ such that $Y-Y^0$ is a normal crossings divisor on $Y$. We describe a logarithmic $F$-crystal on $Y$ whose rational crystalline cohomology is the rigid cohomology of $X$, in particular provides a natural $W[F]$-lattice inside the latter; here $W$ is the Witt vector ring of $k$. If a finite group $G$ acts compatibly on $X$, $Y^0$ and $Y$ then our construction is $G$-equivariant. As an example we apply it to Deligne-Lusztig varieties. For a finite field $k$, if ${\mathbb G}$ is a connected reductive algebraic group defined over $k$ and ${\mathbb L}$ a $k$-rational torus satisfying a certain standard condition, we obtain a meaningful equivariant $W[F]$-lattice in the cohomology ($\ell$-adic or rigid) of the corresponding Deligne-Lusztig variety and an expression of its reduction modulo $p$ in terms of equivariant Hodge cohomology groups.