On two-distance-transitive graphs

Abstract: A $2$-distance-transitive graph is a vertex-transitive graph whose vertex stabilizer is transitive on both the first step and the second step neighborhoods. In this paper, we first answer a question of A. Devillers, M. Giudici, C. H. Li and C. E. Praeger in 2012 about vertex-quasiprimitive $2$-distance-transitive graphs for the odd order case. Then we characterize $2$-distance-transitive graphs of valency $p$ or $p+1$ where $p$ is a prime. After that, as an application of the above result, we classify locally-primitive, $2$-distance-transitive graphs of small valency. In addition to the above results on $2$-distance-transitive graphs, we also classify a family of amply regular graphs with diameter at least $4$ and parameters $(v, k, \lambda, \frac{k - 1}{2})$, and these graphs arise naturally in the classification of locally-primitive, $2$-distance-transitive graphs with small valency.