Equivariant Euler characteristics and sheaf resolvents

Abstract

For certain tame abelian covers of arithmetic surfaces X/Y we obtain striking formulas, involving a quadratic form derived from intersection numbers, for the equivariant Euler characteristics of both the canonical sheaf !X/Y and also its square root !1/2 X/Y . These formulas allow us us to carry out explicit calculations; in particular, we are able to exhibit examples where these two Euler characteristics and that of the the structure sheaf of X are all different and non-trivial. Our results are obtained by using resolvent techniques together with the local Riemann-Roch approach developed in [CPT].