Quadratic Euler Characteristic of Geometrically Cyclic Branched Coverings

Abstract

For an $n$-fold geometrically cyclic branched covering $Y$ of a smooth, projective scheme $X$ branched at a smooth closed subscheme $Z\subset X$ with $n \in k^\times$, we compute the quadratic Euler characteristic of $Y$ in terms of certain Euler classes on $X$ and $Z$ using the quadratic Riemann-Hurwitz formula of Levine. In certain cases with $n$ odd, we relate the quadratic Euler characteristic of $Y$ to the quadratic Euler characteristics of $X$ and $Z$, obtaining similar formulae to the situation in topology. As an application, we compute the quadratic Euler characteristic of geometrically cyclic branched double coverings of $\mathbb{P}^2$.