On the Spectrum of Wenger Graphs

Abstract

Let $q=p^e$, where $p$ is a prime and $e\ge 1$ is an integer. For $m\ge 1$, let $P$ and $L$ be two copies of the $(m+1)$-dimensional vector spaces over the finite field ${\mathbb{F}}_q$. Consider the bipartite graph $W_m(q)$ with partite sets $P$ and $L$ defined as follows: a point $(p)=(p_1,p_2,\dots ,p_{m+1})\in P$ is adjacent to a line $[l]=[l_1,l_2,\dots ,l_{m+1}]\in L$ if and only if the following $m$ equalities hold: $l_{i+1}+p_{i+1}=l_i{p}_1$ for $i=1,\dots ,m$. We call the graphs $W_m(q)$ Wenger graphs. In this paper, we determine all distinct eigenvalues of the adjacency matrix of $W_m(q)$ and their multiplicities. We also survey results on Wenger graphs.

9 pages; accepted for publication to J. Combin. Theory, Series B