Symmetric graphs with 2-arc transitive quotients

Abstract

A graph $\Ga$ is $G$-symmetric if $\Ga$ admits $G$ as a group of automorphisms acting transitively on the set of vertices and the set of arcs of $\Ga$, where an arc is an ordered pair of adjacent vertices. In the case when $G$ is imprimitive on $V(\Ga)$, namely when $V(\Ga)$ admits a nontrivial $G$-invariant partition $\BB$, the quotient graph $\Ga_{\BB}$ of $\Ga$ with respect to $\BB$ is always $G$-symmetric and sometimes even $(G,2)$-arc transitive. (A $G$-symmetric graph is $(G,2)$-arc transitive if $G$ is transitive on the set of oriented paths of length two.) In this paper we obtain necessary conditions for $\Ga_{\BB}$ to be $(G,2)$-arc transitive (regardless of whether $\Ga$ is $(G,2)$-arc transitive) in the case when $v-k$ is an odd prime $p$, where $v$ is the block size of $\BB$ and $k$ is the number of vertices in a block having neighbours in a fixed adjacent block. These conditions are given in terms of $v,k$ and two other parameters with respect to $(\Ga, \BB)$ together with a certain 2-point transitive block design induced by $(\Ga, \BB)$. We prove further that if $p=3$ or $5$ then these necessary conditions are essentially sufficient for $\Ga_{\BB}$ to be $(G,2)$-arc transitive.

To appear in Journal of the Australian Mathematical Society. (The previous title of this paper was "Finite symmetric graphs with two-arc transitive quotients III")