Generalized Artin-Mumford curves over finite fields

Abstract

Let $\mathbb{F}_q$ be the finite field of order $q=p^h$ with $p>2$ prime and $h>1$, and let $\mathbb{F}_{\bar{q}}$ be a subfield of $\mathbb{F}_q$. From any two $\bar{q}$-linearized polynomials $L_1,L_2 \in \overline{\mathbb{F}}_q[T]$ of degree $q$, we construct an ordinary curve $\mathcal{X}_{(L_1,L_2)}$ of genus $(q-1)^2$ which is a generalized Artin-Schreier cover of the projective line $\mathbb{P}^1$. The automorphism group of $\mathcal{X}_{(L_1,L_2)}$ over the algebraic closure $\overline{\mathbb{F}}_q$ of $\mathbb{F}_q$ contains a semidirect product $\Sigma \rtimes \Gamma$ of an elementary abelian $p$-group $\Sigma$ of order $q^2$ by a cyclic group $\Gamma$ of order $\bar{q}-1$. We show that for $L_1 \neq L_2$, $\Sigma \rtimes \Gamma$ is the full automorphism group ${\rm Aut}(\mathcal{X}_{(L_1,L_2)})$ over $\overline{\mathbb{F}}_q$; for $L_1=L_2$ there exists an extra involution and ${\rm Aut}(\mathcal{X}_{(L_1,L_1)})=\Sigma \rtimes \Delta$ with a dihedral group $\Delta$ of order $2(\bar{q}-1)$ containing $\Gamma$. Two different choices of the pair $\{L_1,L_2\}$ may produce birationally isomorphic curves, even for $L_1=L_2$. We prove that any curve of genus $(q-1)^2$ whose $\overline{\mathbb{F}}_q$-automorphism group contains an elementary abelian subgroup of order $q^2$ is birationally equivalent to $\mathcal{X}_{(L_1, L_2)}$ for some separable $\bar{q}$-linearized polynomials $L_1,L_2$ of degree $q$. We produce an analogous characterization in the special case $L_1=L_2$. This extends a result on the Artin-Mumford curves, due to Arakelian and Korchm\'aros.