$p$-group Galois covers of curves in characteristic $p$. II

Abstract

Let $k$ be an algebraically closed field of characteristic $p>0$ and let $G$ be a finite $p$-group. The results of Harbater, Katz and Gabber associate to every $k$-linear action of $G$ on $k[[t]]$ an HKG-cover, i.e. a $G$-cover of the projective line ramified only over $\infty$. In this paper we relate the HKG-covers to the classical problem of determining the equivariant structure of cohomologies of a curve with an action of $G$. To this end, we present a new way of computing cohomologies of HKG-covers. As an application of our results, we compute the equivariant structure of the de Rham cohomology of Klein four covers in characteristic $2$.