Automorphisms of a Distance-Regular Graph with Intersection Array $\{30,22,9;1,3,20\}$

K. S. Efimov ; A. A. Makhnev

Trudy Instituta matematiki i mehaniki, Trudy Instituta Matematiki i Mekhaniki UrO RAN, Tome 26 (2020) no. 3, pp. 23-31

Résumé

A distance-regular graph $\Gamma$ of diameter 3 is called a Shilla graph if it has the second eigenvalue $\theta_1=a_3$. In this case $a=a_3$ divides $k$ and we set $b=b(\Gamma)=k/a$. Koolen and Park obtained the list of intersection arrays for Shilla graphs with $b=3$. There exist graphs with intersection arrays $\{12,10,5;1,1,8\}$ and $\{12,10,3;1,3,8\}$. The nonexistence of graphs with intersection arrays $\{12,10,2;1,2,8\}$, $\{27,20,10;1,2,18\}$, $\{42,30,12;1,6,28\}$, and $\{105,72,24;1,12,70\}$ was proved earlier. In this paper, we study the automorphisms of a distance-regular graph $\Gamma$ with intersection array $\{30,22,9;1,3,20\}$, which is a Shilla graph with $b=3$. Assume that $a$ is a vertex of $\Gamma$, $G={\rm Aut}(\Gamma)$ is a nonsolvable group, $\bar G=G/S(G)$, and $\bar T$ is the socle of $\bar G$. Then $\bar T\cong L_2(7)$, $A_7$, $A_8$, or $U_3(5)$. If $\Gamma$ is arc-transitive, then $T$ is an extension of an irreducible $F_2U_3(5)$-module $V$ by $U_3(5)$ and the dimension of $V$ over $F_3$ is 20, 28, 56, 104, or 288.

Détail

Keywords: Shilla graph
Mots-clés : graph automorphism.