Determining the vertex stabilizers of 4-valent half-arc-transitive graphs

Abstract

We say that a group is a $4$-HAT-stabilizer if it is the vertex stabilizer of some connected $4$-valent half-arc-transitive graph. In 2001, Marušič and Nedela proved that every $4$-HAT-stabilizer must be a concentric group. However, over the past two decades, only a very small proportion of concentric groups have been shown to be $4$-HAT-stabilizers. This paper develops a theory that provides a general framework for determining whether a concentric group is a $4$-HAT-stabilizer. With this approach, we significantly extend the known list of $4$-HAT-stabilizers. As a corollary, we confirm that ${\mathcal{H}}_7\times C_2^{m-7}$ are $4$-HAT-stabilizers for $m\ge 7$, achieving the goal of a conjecture posed by Spiga and Xia.