Curves with a large automorphism group admitting a cyclic subgroup of index $2$

Abstract

The Hurwitz bound on the order of the $\mathbb{K}$-automorphism group $\mathrm{Aut}(\mathcal{X})$ of an algebraic curve $\mathcal{X}$ of genus $g(\mathcal{X})\ge 2$ defined over a field $\mathbb{K}$ of zero characteristic states that $|\mathrm{Aut}(\mathcal{X})|\le 84(g(\mathcal{X})-1)$. Improved bounds are available for the order of certain types of subgroups within automorphism groups. For instance, if a subgroup $H$ of $\mathrm{Aut}(\mathcal{X})$ is dihedral, then in the complex case, $|H|\le 4g(\mathcal{X})+4$. More recently it has been shown that a tighter bound holds for $H$ a generalized quasi-dihedral group. In this paper we explore the more general setting of a curve defined over a field of any characteristic, and $H$ a group admitting a cyclic subgroup of index two. We show that the same upper bound for the size of a dihedral group of automorphisms holds for curves defined over an algebraically closed field of characteristic $p\ne 2$. Then we provide some classification results about (non-dihedral) groups of size larger than $4g(\mathcal{X})+4$ admitting a cyclic subgroup of index $2$.