Soukal, Roman, and Přemysl Holub. “A Note on Packing Chromatic Number of the Square Lattice”. The Electronic Journal of Combinatorics 17, no. 1 (March 15, 2010): #N17. Accessed May 9, 2026. https://www.combinatorics.org/ojs/index.php/eljc/article/view/v17i1n17.