Distance-Regular Graphs with a Relatively Small Eigenvalue Multiplicity

Abstract

Godsil showed that if $\Gamma$ is a distance-regular graph with diameter $D \geq 3$ and valency $k \geq 3$, and $\theta$ is an eigenvalue of $\Gamma$ with multiplicity $m \geq 2$, then $k \leq\frac{(m+2)(m-1)}{2}$.

In this paper we will give a refined statement of this result. We show that if $\Gamma$ is a distance-regular graph with diameter $D \geq 3$, valency $k \geq 2$ and an eigenvalue $\theta$ with multiplicity $m\geq 2$, such that $k$ is close to $\frac{(m+2)(m-1)}{2}$, then $\theta$ must be a tail. We also characterize the distance-regular graphs with diameter $D \geq 3$, valency $k \geq 3$ and an eigenvalue $\theta$ with multiplicity $m \geq 2$ satisfying $k= \frac{(m+2)(m-1)}{2}$.