Inseparable maps on $W_n$-valued local cohomology groups of non-taut rational double point singularities and the height of K3 surfaces

Abstract

We consider rational double point singularities (RDPs) that are non-taut, which means that the isomorphism class is not uniquely determined from the dual graph of the minimal resolution. Such RDPs exist in characteristic $2,3,5$. We compute the actions of Frobenius, and other inseparable morphisms, on $W_n$-valued local cohomology groups of RDPs. Then we consider RDP K3 surfaces admitting non-taut RDPs. We show that the height of the K3 surface, which is also defined in terms of the Frobenius action on $W_n$-valued cohomology groups, is related to the isomorphism class of the RDP.

28 pages. Accepted version. Main results unchanged. For a morphism $f$ between RDP K3 surfaces, the notion called the height of $f$ in the older versions is now called 1 + the gap of $f$