The classification of extremely primitive groups

TL;DR

This paper classifies all finite extremely primitive groups, especially those with socle an exceptional Lie type, completing the broader classification of such groups and providing new insights into their structure and actions.

Contribution

It determines the almost simple extremely primitive groups with exceptional Lie type socles, completing their classification and establishing new results on base sizes for these groups.

Findings

Complete classification of almost simple extremely primitive groups with exceptional Lie type socles.

New results on base sizes for primitive actions of exceptional groups.

Finiteness of exceptions in the classification, with a conjecture of none remaining.

Abstract

Let $G$ be a finite primitive permutation group on a set $\Omega$ with nontrivial point stabilizer $G_{\alpha}$. We say that $G$ is extremely primitive if $G_{\alpha}$ acts primitively on each of its orbits in $\Omega \setminus \{\alpha\}$. These groups arise naturally in several different contexts and their study can be traced back to work of Manning in the 1920s. In this paper, we determine the almost simple extremely primitive groups with socle an exceptional group of Lie type. By combining this result with earlier work of Burness, Praeger and Seress, this completes the classification of the almost simple extremely primitive groups. Moreover, in view of results by Mann, Praeger and Seress, our main theorem gives a complete classification of all finite extremely primitive groups, up to finitely many affine exceptions (and it is conjectured that there are no exceptions). Along the way, we also establish several new results on base sizes for primitive actions of exceptional groups, which may be of independent interest.