Colouring $(P_r+P_s)$-Free Graphs

TL;DR

This paper investigates the complexity of 3-Colouring and List 3-Colouring problems on graphs that do not contain certain disjoint unions of paths, providing polynomial-time solutions for specific cases and a complete classification for small graphs.

Contribution

It proves polynomial-time solvability of List 3-Colouring for (P2+P5)-free and (P3+P4)-free graphs, completing the complexity classification for all H-free graphs with up to seven vertices.

Findings

List 3-Colouring is polynomial-time solvable for (P2+P5)-free graphs.

List 3-Colouring is polynomial-time solvable for (P3+P4)-free graphs.

Complete complexity classifications for 3-Colouring and List 3-Colouring on H-free graphs up to seven vertices.

Abstract

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 $L(u) \subseteq \{1,\cdots, k\}$, 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. The graph $P_r+P_s$ is the disjoint union of the $r$-vertex path $P_r$ and the $s$-vertex path $P_s$. 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.