Eigenvalues and eigenfunctions of a Hamming ball

Abstract

We describe the eigenvalues and the eigenspaces of the adjacency matrices of subgraphs of the Hamming cube induced by Hamming balls, and more generally, by a union of adjacent concentric Hamming spheres. As a corollary, we extend the range of cardinalities of subsets of the Hamming cube for which Hamming balls have essentially the largest maximal eigenvalue (among all subsets of the same size). We show that this holds even when the sets in question are large, with cardinality which is an arbitrary subconstant fraction of the whole cube.