Complete multipartite graphs that are determined, up to switching, by their Seidel spectrum

Abstract

It is known that complete multipartite graphs are determined by their distance spectrum but not by their adjacency spectrum. The Seidel spectrum of a graph G on more than one vertex does not determine the graph, since any graph obtained from G by Seidel switching has the same Seidel spectrum. We consider G to be determined by its Seidel spectrum, up to switching, if any graph with the same spectrum is switching equivalent to a graph isomorphic to G. It is shown that any graph which has the same spectrum as a complete k-partite graph is switching equivalent to a complete k-partite graph, and if the different partition sets sizes are p1,…,pl, and there are at least three partition sets of each size pi, i=1,…,l, then G is determined, up to switching, by its Seidel spectrum. Sufficient conditions for a complete tripartite graph to be determined by its Seidel spectrum are discussed.