On subsets of the hypercube with prescribed Hamming distances

A celebrated theorem of Kleitman in extremal combinatorics states that a collection of binary vectors in {0,1}n with diameter d has cardinality at most that of a Hamming ball of radius d/2. In this paper, we give an algebraic proof of Kleitman's Theorem, by carefully choosing a pseudo-adjacency matrix for certain Hamming graphs, and applying the Cvetković bound on independence numbers. This method also allows us to prove several extensions and generalizations of Kleitman's Theorem to other allowed distance sets, in particular blocks of consecutive integers of width much smaller than n. We also improve on a theorem of Alon about subsets of Fpn whose difference set does not intersect {0,1}n nontrivially.