A Note on Packing Chromatic Number of the Square Lattice  Roman
Soukal, Přemysl Holub   The concept of a packing colouring is
related to a frequency assignment problem. The packing chromatic
number $\chi_p(G)$ of a graph $G$ is the smallest integer $k$
such that the vertex set $V (G)$ can be partitioned into
disjoint classes $X_1, \dots, X_k$, where vertices in $X_i$ have
pairwise distance greater than $i$. In this note we improve the
upper bound on the packing chromatic number of the square
lattice.  Full Text: PDF