Bases of quasisimple linear groups

TL;DR

This paper proves that quasisimple linear groups acting irreducibly on a vector space generally have a base of size at most 6, with two specific families of exceptions involving alternating and classical groups.

Contribution

It establishes a uniform bound on the base size for quasisimple linear groups, excluding two well-characterized families, advancing understanding of their permutation properties.

Findings

Most quasisimple linear groups have base size ≤ 6.

Two exceptional families are identified involving alternating and classical groups.

The result applies to groups acting irreducibly on finite vector spaces.

Abstract

Let $V$ be a vector space of dimension $d$ over $F_q$, a finite field of $q$ elements, and let $G \le GL(V) \cong GL_d(q)$ 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 ${\rm Alt}_m$ 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 $F_q$.