On the crystalline cohomology of Deligne-Lusztig varieties

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.