On Solvable Automorphism Groups of Function Fields over Finite Fields

Abstract

The classical Hurwitz bound gives a linear upper bound in terms of the genus for automorphism groups of algebraic function fields in characteristic zero, whereas algebraic function fields in positive characteristic can have much larger automorphism groups. Usually, one considers the case where the constant field is an algebraically closed field. Here we focus on function fields over a finite constant field. It turns out that the situation is very different. We firstly investigate solvable subgroups and $p$-subgroups of $\mathrm{Aut}(F/{\mathbb{F}}_{\mathrm{q}})$, where $p=\mathrm{char}({\mathbb{F}}_{\mathrm{q}})$ and $F/{\mathbb{F}}_q$ is a function field of genus $g_F\ge 2$ with full constant field ${\mathbb{F}}_q$. We prove that there are explicit constants $C_q^{(sol)}$ and $C_q^{(p)}$, depending only on $q$, such that, for every solvable subgroup $G\le \mathrm{Aut}(F/{\mathbb{F}}_{\mathrm{q}})$ and every $p$-subgroup $P\le \mathrm{Aut}(F/{\mathbb{F}}_{\mathrm{q}})$, $$|G|\le C_q^{(sol)}(g_F-1),|P|\le C_q^{(p)}(g_F-1).$$ In particular, one may take $C_q^{(sol)}=12q^2(q-1)$ for $q\ge 4$ and $C_q^{(p)}=5q$ for every $q$. In the second part of this article, we give asymptotic lower bounds for solvable automorphism groups. For every $q$ there is an explicit constant $L_q>0$ and a sequence of function fields $F_i/{\mathbb{F}}_q$ with $g_{F_i}\to \infty$, together with solvable subgroups $G_i\le \mathrm{Aut}({\mathrm{F}}_{\mathrm{i}}/{\mathbb{F}}_{\mathrm{q}})$, such that $$|G_i|\ge L_q(g_{F_i}-1)$$ for all $i$. For $q\ge 5$ one may take $L_q=2q(q-1)/(q-2)$. If $q=r^2$ with $r\ge 3$, an unramified tower construction over the Hermitian function field gives the stronger choice $L_q=2r^3(r-1)/(r-2)$. Moreover, as an application of our tower construction above, we improve the asymptotic lower bound of Ma and Xing for subgroups of $\mathrm{Aut}(F/{\mathbb{F}}_{\mathrm{q}})$ whose order is relatively prime to $p$. As another application, we extend our method to obtain explicit linear bounds for $p$-solvable groups in terms of the genus.

arXiv comment25 pages