Strongly cospectral vertices in normal Cayley graphs

Abstract

We prove an upper bound on the number of pairwise strongly cospectral vertices in a normal Cayley graph, in terms of the multiplicities of its eigenvalues. We use this to determine an explicit bound in Cayley graphs of $\mathbb{Z}_2^d$ and $\mathbb{Z}_4^d$. We also provide some infinite families of Cayley graphs of $\mathbb{Z}_2^d$ with a set of four pairwise strongly cospectral vertices and show that such graphs exist in every dimension.