  The competition graph of a digraph D is a graph which has the same vertex set
as D and has an edge between x and y if and only if there exists a vertex v in
D such that (x,v) and (y,v) are arcs of D. For any graph G, G together with
sufficiently many isolated vertices is the competition graph of some acyclic
digraph. The competition number k(G) of a graph G is defined to be the smallest
number of such isolated vertices. In general, it is hard to compute the
competition number k(G) for a graph G and it has been one of important research
problems in the study of competition graphs. In this paper, we compute the
competition numbers of Hamming graphs with diameter at most three.
