Bounds on Special Subsets in Graphs, Eigenvalues and Association Schemes

Abstract

We give a bound on the sizes of two sets of vertices at a given minimum distance in a graph in terms of polynomials and the Laplace spectrum of the graph. We obtain explicit bounds on the number of vertices at maximal distance and distance two from a given vertex, and on the size of two equally large sets at maximal distance. For graphs with four eigenvalues we find bounds on the number of vertices that are not adjacent to a given vertex and that have μ common neighbours with that vertex. Furthermore we find that the regular graphs for which the bounds are tight come from association schemes.