Creative Commons Attribution 3.0 Unported license
The k-Colouring problem is to decide if the vertices of a graph can be coloured with at most k colours for a fixed integer k such that no two adjacent vertices are coloured alike. If each vertex u must be assigned a colour from a prescribed list L(u) subseteq {1,...,k}, then we obtain the List k-Colouring problem. A graph G is H-free if G does not contain H as an induced subgraph. We continue an extensive study into the complexity of these two problems for H-free graphs. We prove that List 3-Colouring is polynomial-time solvable for (P_2+P_5)-free graphs and for (P_3+P_4)-free graphs. Combining our results with known results yields complete complexity classifications of 3-Colouring and List 3-Colouring on H-free graphs for all graphs H up to seven vertices. We also prove that 5-Colouring is NP-complete for (P_3+P_5)-free graphs.
@InProceedings{klimosova_et_al:LIPIcs.ISAAC.2018.5,
author = {Klimosov\'{a}, Tereza and Mal{\'\i}k, Josef and Masar{\'\i}k, Tom\'{a}s and Novotn\'{a}, Jana and Paulusma, Dani\"{e}l and Sl{\'\i}vov\'{a}, Veronika},
title = {{Colouring (P\underliner+P\underlines)-Free Graphs}},
booktitle = {29th International Symposium on Algorithms and Computation (ISAAC 2018)},
pages = {5:1--5:13},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-094-1},
ISSN = {1868-8969},
year = {2018},
volume = {123},
editor = {Hsu, Wen-Lian and Lee, Der-Tsai and Liao, Chung-Shou},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2018.5},
URN = {urn:nbn:de:0030-drops-99533},
doi = {10.4230/LIPIcs.ISAAC.2018.5},
annote = {Keywords: vertex colouring, H-free graph, linear forest}
}