On the classification problem for the genera of quotients of the Hermitian curve

Abstract

In this paper we characterize the genera of those quotient curves ${\mathcal{H}}_q/G$ of the ${\mathbb{F}}_{q^2}$-maximal Hermitian curve ${\mathcal{H}}_q$ for which $G$ is contained in the maximal subgroup ${\mathcal{M}}_1$ of $\mathrm{Aut}({\mathcal{H}}_q)$ fixing a self-polar triangle, or $q$ is even and $G$ is contained in the maximal subgroup ${\mathcal{M}}_2$ of $\mathrm{Aut}({\mathcal{H}}_q)$ fixing a pole-polar pair $(P,\ell )$ with respect to the unitary polarity associated to ${\mathcal{H}}_q({\mathbb{F}}_{q^2})$. In this way several new values for the genus of a maximal curve over a finite field are obtained. Together with what is known in the literature, our results leave just two open cases to provide the complete list of genera of Galois subcovers of the Hermitian curve; namely, the open cases in [Bassa-Ma-Xing-Yeo, J. Combin. Theory Ser. A, 2013] when $G$ fixes a point $P\in {\mathcal{H}}_q({\mathbb{F}}_{q^2})$ and $q$ is even, and the open cases in [Montanucci-Zini, Comm. Algebra, 2018] when $G\le {\mathcal{M}}_2$ and $q$ is odd.