The Hamming distance dist(u, v) between two binary vectors v= (v1,...,vn) and w= (w1,...,wn) is the number of indices k such that vk wk. A fundamental question in coding theory is to determine the number A(n, d) = max|{ S {0,1}n | dist(u, v) d for all distinct u, v S }|, the maximal number of binary vectors of length n that one can find such that any two distinct vectors have a Hamming distance d. For example, A(5,4) = 2.