List 3-Coloring Graphs with No Induced P6+rP3

This is a post-peer-review, pre-copyedit version of an article published in Algorithmica. The final authenticated version is available online at: https://doi.org/10.1007/s00453-020-00754-yFor an integer t, we let Pt denote the t-vertex path. We write H+G for the disjoint union of two graphs H and G, and for an integer r and a graph H, we write rH for the disjoint union of r copies of H. We say that a graph G is H-free if no induced subgraph of G is isomorphic to the graph H. In this paper, we study the complexity of k-coloring, for a fixed integer k, when restricted to the class of H-free graphs with a fixed graph H. We provide a polynomial-time algorithm to test if, for fixed r, a (P6+rP3)-free is three-colorable, and find a coloring if one exists. We also solve the list version of this problem, where each vertex is assigned a list of possible colors, which is a subset of {1,2,3}. This generalizes results of Broersma, Golovach, Paulusma, and Song, and results of Klimošová, Malik, Masařík, Novotná, Paulusma, and Slívová. Our proof uses a result of Ding, Seymour, and Winkler relating matchings and hitting sets in hypergraphs. We also prove that the problem of deciding if a (P5+P2)-free graph has a k-coloring is NP-hard for every fixed k≥5.The first author was supported by NSF Grant DMS-1550991. This material is based upon work supported in part by the U. S. Army Research Laboratory and the U. S. Army Research Office under Grant number W911NF-16-1-0404. The second author was supported by the National Natural Science Foundation of China (11801284)