On two-distance-transitive graphs

TL;DR

This paper investigates the structure and classification of 2-distance-transitive graphs, answering open questions, characterizing specific valencies, and classifying related graphs with applications to amply regular graphs.

Contribution

It provides new classifications and characterizations of 2-distance-transitive graphs, including solutions to open problems and analysis of graphs with prime valency and small valency.

Findings

Solved a question on vertex-quasiprimitive 2-distance-transitive graphs of odd order.

Characterized 2-distance-transitive graphs with prime or prime+1 valency.

Classified a family of amply regular graphs with diameter at least 4.

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.