The minimum genus of Galois covers of curves

Abstract

Let $Y\longrightarrow X$ be a $G$-Galois connected cover of smooth projective curves over an algebraically closed field $k$ of positive characteristic $p$ with the branch locus contained in a finite subset of closed points $S_X$ in $X$, where $G$ is a finite cyclic $p$-group and $l$ is a prime number other than $p$. Let $\Gamma$ be an extension of an elementary abelian $l$-group $H$ by $G$. We find $G$-stable submodules of $l$-torsion of the Picard group of $Y$ and its generalisation. This is used to describe a method for finding the minimum genus of $\Gamma$-covers of $X$, \'etale over $X\setminus S_X$, dominating $Y$; and also the minimum of the genera of $\Gamma$-covers of $\mathbb{P}^1$ \'etale over $\mathbb{A}^1$.