A K3-WORM coloring of a graph G is an assignment of colors to the vertices in such a way that the vertices of each K3-subgraph of G get precisely two colors. We study graphs G which admit at least one such coloring. We disprove a conjecture of Goddard et al. [Congr. Numer. 219 (2014) 161-173] by proving that for every integer k ≥ 3 there exists a K3-WORM-colorable graph in which the minimum number of colors is exactly k. There also exist K3-WORM colorable graphs which have a K3-WORM coloring with two colors and also with k colors but no coloring with any of 3, . . . , k − 1 colors. We also prove that it is NP-hard to determine the minimum number of colors, and NP-complete to decide k-colorability for every k ≥ 2 (and remains intractable even for graphs of maximum degree 9 if k = 3). On the other hand, we prove positive results for d-degenerate graphs with small d, also including planar graphs.
@article{bwmeta1.element.doi-10_7151_dmgt_1891,
author = {Csilla Bujt\'as and Zsolt Tuza},
title = {K3-Worm Colorings of Graphs: Lower Chromatic Number and Gaps in the Chromatic Spectrum},
journal = {Discussiones Mathematicae Graph Theory},
volume = {36},
year = {2016},
pages = {759-772},
zbl = {1339.05116},
language = {en},
url = {http://dml.mathdoc.fr/item/bwmeta1.element.doi-10_7151_dmgt_1891}
}
Csilla Bujtás; Zsolt Tuza. K3-Worm Colorings of Graphs: Lower Chromatic Number and Gaps in the Chromatic Spectrum. Discussiones Mathematicae Graph Theory, Tome 36 (2016) pp. 759-772. http://gdmltest.u-ga.fr/item/bwmeta1.element.doi-10_7151_dmgt_1891/
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