Labelling Graphs with a Condition at Distance 2

Given a simple graph G (V, E) and a positive number d, an Ld(2, 1)-labelling of G is a function f V(G) [0, oc) such that whenever x, y E V are adjacent, If(x)f(Y)l >2d, and whenever the distance between x and y is two, If(x) f(Y)l >d. The Ld(2, 1)-labelling number A(G, d) is the smallest number m such that G has an Ld(2, 1)-labelling f with max{f(v) v E V} m. It is shown that to determine A(G, d), it suffices to study the case when d 1 and the labelling is nonnegative integral-valued. Let A(G) A(G, 1). The labelling numbers of special classes of graphs, e.g., A(C) 4 for any cycle C, are described. It is shown that for graphs of maximum degree A, A(G) _ A + 2A. If G is diameter 2, A(G) _ A2, a sharp bound for some A. Determining A(G) is shown to be NP-complete by relating it to the problem of finding Hamilton paths.