On \((s,t)\)-Relaxed \(L(2,1)\)-Labelings of the Hexagonal Lattice

Abstract

Suppose \(G\) is a graph. Let \(u\) be a vertex of \(G\). A vertex \(v\) is called an \(i\)-neighbor of \(u\) if \(d_G(u,v) = i\). A \(1\)-neighbor of \(u\) is simply called a neighbor of \(u\). Let \(s\) and \(t\) be two nonnegative integers. Suppose \(f\) is an assignment of nonnegative integers to the vertices of \(G\). If the following three conditions are satisfied, then \(f\) is called an \((s, t)\)-relaxed \(L(2,1)\)-labeling of \(G\): (1) for any two adjacent vertices \(u\) and \(v\) of \(G\), \(f(u) \neq f(v)\); (2) for any vertex \(u\) of \(G\), there are at most \(s\) neighbors of \(u\) receiving labels from \(\{f(u) – 1, f(u)+ 1\}\); (3) for any vertex \(u\) of \(G\), the number of \(2\)-neighbors of \(u\) assigned the label \(f(u)\) is at most \(t\). The minimum span of \((s, t)\)-relaxed \(L(2,1)\)-labelings of \(G\) is called the \((s,t)\)-relaxed \(L(2,1)\)-labeling number of \(G\), denoted by \(\lambda_{2,1}^{s,t}(G)\). It is clear that \(\lambda_{2,1}^{0,0}(G)\) is the so-called \(L(2, 1)\)-labeling number of \(G\). In this paper, the \((s, t)\)-relaxed \(L(2, 1)\)-labeling number of the hexagonal lattice is determined for each pair of two nonnegative integers \(s\) and \(t\). And this provides a series of channel assignment schemes for the corresponding channel assignment problem on the hexagonal lattice.