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

Abstract

Let $Γ$ be a finite X-symmetric graph with a nontrivial X-invariant partition $\mathcal {B}$ on $V(Γ)$ such that $Γ_{\mathcal {B}}$ is a connected (X,2)-arc-transitive graph and $Γ$ is not a multicover of $Γ_{\mathcal {B}}$. This article aims to give a characterization of $(Γ, X, \mathcal {B})$ for the case where $|Γ(C) \cap B| = 3$ for $B\in \mathcal {B}$ and $C \in Γ_{\mathcal {B}}(B)$. 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 $Σ$ can occur as $Γ_{\mathcal {B}}$ if and only if $X_τ^{Σ(τ)}\cong PSL(3,2)$ for $τ\in V(Σ)$.

22 pages