Tetravalent $2$-arc-transitive Cayley graphs on non-abelian simple groups

TL;DR

This paper classifies tetravalent 2-arc-transitive Cayley graphs on non-abelian simple groups, reducing the possible groups from 22 to 7 and detailing the automorphism groups' structure.

Contribution

It narrows down the classification of such graphs by identifying only 7 possible groups and describing their automorphism groups explicitly.

Findings

Number of candidate groups reduced from 22 to 7

For each candidate, automorphism group contains a normal simple subgroup

Explicit pairings of groups G and T are provided

Abstract

A graph Gamma is said to be 2-arc-transitive if its full automorphism group Aut(\Gamma) has a single orbit on ordered paths of length 2, and for G\leq Aut(\Gamma), \Gamma is G-regular if G is regular on the vertex set of \Gamma. Let G be a finite non-abelian simple group and let \Gamma be a connected tetravalent 2-arc-transitive G-regular graph. In 2004, Fang, Li and Xu proved that either G\unlhd \Aut(\Gamma) or G is one of 22 possible candidates. In this paper, the number of candidates is reduced to 7, and for each candidate G, it is shown that \Aut(\Gamma) has a normal arc-transitive non-abelian simple subgroup T such that G\leq T and the pair (G,T) is explicitly given