The /spl Delta//sup 2/-conjecture for L(2,1)-labelings is true for direct and strong products of graphs

A variation of the channel-assignment problem is naturally modeled by L(2,1)-labelings of graphs. An L(2,1)-labeling of a graph G is an assignment of labels from {0,1,...,/spl lambda/} to the vertices of G such that vertices at distance two get different labels and adjacent vertices get labels that are at least two apart and the /spl lambda/-number /spl lambda/(G) of G is the minimum value /spl lambda/ such that G admits an L(2,1)-labeling. The /spl Delta//sup 2/-conjecture asserts that for any graph G its /spl lambda/-number is at most the square of its largest degree. In this paper it is shown that the conjecture holds for graphs that are direct or strong products of nontrivial graphs. Explicit labelings of such graphs are also constructed.