Totally 22-closed finite groups with trivial Fitting subgroup
Abstract
A finite permutation group G≤Sym(Ω)G≤Sym(Ω) is called 22-closed if GG is the largest subgroup of Sym(Ω)Sym(Ω) which leaves invariant each of the GG-orbits for the induced action on Ω×ΩΩ×Ω. Introduced by Wielandt in 1969, the concept of 22-closure has developed as one of the most useful approaches for studying relations on a finite set invariant under a group of permutations of the set; in particular for studying automorphism groups of graphs and digraphs. The concept of total 22-closure switches attention from a particular group action, and is a property intrinsic to the group: a finite group GG is said to be totally 22-closed if GG is 22-closed in each of its faithful permutation representations. There are infinitely many finite soluble totally 22-closed groups, and these have been completely characterized, but up to now no insoluble examples were known. It turns out, somewhat surprisingly to us, that there are exactly 66 totally 22-closed finite nonabelian simple groups: the Janko groups J1J1, J3J3 and J4J4, together with LyLy, ThTh and the Monster 𝕄. Moreover, if a finite totally 2-closed group has no nontrivial abelian normal subgroup, then we show that it is a direct product of some (but not all) of these simple groups, and there are precisely 47 examples.
In the course of obtaining this classification, we develop a general framework for studying 2-closures of transitive permutation groups, which we hope will prove useful for investigating representations of finite groups as automorphism groups of graphs and digraphs, and in particular for attacking the long-standing polycirculant conjecture. In this direction, we apply our results, proving a dual to a 1939 theorem of Frucht from Algebraic Graph Theory. We also pose several open questions concerning closures of permutation groups.
Communicated by Efim Zelmanov