Bases of quasisimple linear groups

Abstract

Let $V$ be a vector space of dimension $d$ over $F_q$, a finite field of $q$ elements, and let $G\le GL(V)\cong G{L}_d(q)$ be a linear group. A base of $G$ is a set of vectors whose pointwise stabiliser in $G$ is trivial. We prove that if $G$ is a quasisimple group (i.e. $G$ is perfect and $G/Z(G)$ is simple) acting irreducibly on $V$, then excluding two natural families, $G$ has a base of size at most 6. The two families consist of alternating groups ${\mathrm{Alt}}_m$ acting on the natural module of dimension $d=m-1$ or $m-2$, and classical groups with natural module of dimension $d$ over subfields of $F_q$.