Prime-valent Symmetric graphs with a quasi-semiregular automorphism

TL;DR

This paper classifies prime-valent symmetric graphs with a special automorphism called quasi-semiregular, revealing their structure as Cayley graphs or covers, and constructs new examples with complex automorphism groups.

Contribution

It provides a classification of prime-valent symmetric graphs with quasi-semiregular automorphisms, including a complete case for p=5 and new infinite families with insoluble automorphism groups.

Findings

Graphs are either Cayley graphs with specific automorphisms or normal covers of simpler graphs.

Complete classification for p=5 with solvable or non-abelian simple automorphism groups.

Construction of the first infinite family with insoluble automorphism groups.

Abstract

An automorphism of a graph is called quasi-semiregular if it fixes a unique vertex of the graph and its remaining cycles have the same length. This kind of symmetry of graphs was first investigated by Kutnar, Malni\v{c}, Mart\'{i}nez and Maru\v{s}i\v{c} in 2013, as a generalization of the well-known semiregular automorphism of a graph. Symmetric graphs of valency three or four, admitting a quasi-semiregular automorphism, have been classified in recent two papers. Let $p\geq 5$ be a prime and $\Gamma$ a connected symmetric graph of valency $p$ admitting a quasi-semiregular automorphism. In this paper, we first prove that either $\Gamma$ is a connected Cayley graph $\rm{Cay}(M,S)$ such that $M$ is a $2$-group admitting a fixed-point-free automorphism of order $p$ with $S$ as an orbit of involutions, or $\Gamma$ is a normal $N$-cover of a $T$-arc-transitive graph of valency $p$ admitting a quasi-semiregular automorphism, where $T$ is a non-abelian simple group and $N$ is a nilpotent group. Then in case $p=5$, we give a complete classification of such graphs $\Gamma$ such that either $\rm{Aut}(\Gamma)$ has a solvable arc-transitive subgroup or $\Gamma$ is $T$-arc-transitive with $T$ a non-abelian simple group. We also construct the first infinite family of symmetric graphs that have a quasi-semiregular automorphism and an insolvable full automorphism group.