Maximal $\mathrm{PSL_2}$ subgroups of exceptional groups of Lie type

TL;DR

This paper classifies maximal subgroups isomorphic to PSL_2(p^a) within exceptional groups of Lie type, showing most are fixed points of algebraic subgroups, with a few cases still unresolved.

Contribution

It proves that most maximal PSL_2(p^a) subgroups are known, except for three specific cases, advancing the classification of subgroup structures in exceptional groups.

Findings

Most maximal PSL_2(p^a) subgroups are fixed points of algebraic A_1 subgroups.

Three cases (p^a=7,8,25 for E_7) remain unresolved but are well-understood.

The classification aligns with recent results, completing the subgroup landscape for these groups.

Abstract

We study embeddings of $\mathrm{PSL}_2(p^a)$ into exceptional groups $G(p^b)$ for $G=F_4,E_6,{}^2\!E_6,E_7$, and $p$ a prime with $a,b$ positive integers. With a few possible exceptions, we prove that any almost simple group with socle $\mathrm{PSL}_2(p^a)$, that is maximal inside an almost simple exceptional group of Lie type $F_4$, $E_6$, ${}^2\!E_6$ and $E_7$, is the fixed points under the Frobenius map of a corresponding maximal closed subgroup of type $A_1$ inside the algebraic group. Together with a recent result of Burness and Testerman for $p$ the Coxeter number plus one, this proves that all maximal subgroups with socle $\mathrm{PSL}_2(p^a)$ inside these finite almost simple groups are known, with three possible exceptions ($p^a=7,8,25$ for $E_7$). In the three remaining cases we provide considerable information about a potential maximal subgroup.