A class of symmetric graphs with 2-arc-transitive quotients

Abstract

Let $\Gamma$ be a finite X-symmetric graph with a nontrivial X-invariant partition $\mathcal {B}$ on $V(\Gamma)$ such that $\Gamma_{\mathcal {B}}$ is a connected (X,2)-arc-transitive graph and $\Gamma$ is not a multicover of $\Gamma_{\mathcal {B}}$. This article aims to give a characterization of $(\Gamma, X, \mathcal {B})$ for the case where $|\Gamma(C) \cap B| = 3$ for $B\in \mathcal {B}$ and $C \in \Gamma_{\mathcal {B}}(B)$. This investigation requires a study on (X,2)-arc-transitive graphs of valency 4 or 7. We give a characterization of tetravalent (X,2)-arc-transitive graphs at first; and as a byproduct, we prove that every tetravalent (X,2)-transitive graph is either the complete graph on 5 vertices or a near n-gonal graph for some $n\ge 4$. Then we show that a heptavalent $(X,2)$-arc-transitive graph $\Sigma$ can occur as $\Gamma_{\mathcal {B}}$ if and only if $X_\tau^{\Sigma(\tau)}\cong PSL(3,2)$ for $\tau\in V(\Sigma)$.