Douglas F. Rall Bostjan Bresar Art S. Finbow Sandi Klavzar On the Packing Chromatic Number of Trees, Cartesian Products and Some Infinite Graphs. 57-61 2008 30 Electronic Notes in Discrete Mathematics http://dx.doi.org/10.1016/j.endm.2008.01.011 db/journals/endm/endm30.html#RallBFK08