The classification of extremely primitive groups

Dedicated to the memory of Jan Saxl

Let G be a finite primitive permutation group on a set Ω with nontrivial point stabilizer Gα⁠. We say that G is extremely primitive if Gα acts primitively on each of its orbits in Ω∖{α}⁠. 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.