The Complexity of (P3, H)-Arrowing and Beyond

TL;DR

This paper investigates the computational complexity of the $(P_3, H)$-Arrowing problem in graph Ramsey theory, establishing coNP-completeness for most cases and polynomial-time solvability when $H=K_3$, and introduces new methods for analyzing these problems.

Contribution

The paper proves the coNP-completeness of $(P_3, H)$-Arrowing for most graphs $H$, introduces a new graph invariant for reductions, and extends hardness results to broader classes of $(F, H)$-Arrowing problems.

Findings

$(P_3, H)$-Arrowing is coNP-complete for all 2-connected $H$ except $K_3$

$(P_3, K_3)$-Arrowing is solvable in polynomial time

New graph invariant aids in constructing reductions and hardness proofs

Abstract

Often regarded as the study of how order emerges from randomness, Ramsey theory has played an important role in mathematics and computer science, giving rise to applications in numerous domains such as logic, parallel processing, and number theory. The core of graph Ramsey theory is arrowing: For fixed graphs $F$ and $H$, the $(F, H)$-Arrowing problem asks whether a given graph, $G$, has a red/blue coloring of the edges of $G$ such that there are no red copies of $F$ and no blue copies of $H$. For some cases, the problem has been shown to be coNP-complete, or solvable in polynomial time. However, a more systematic approach is needed to categorize the complexity of all cases. We focus on $(P_3, H)$-Arrowing as $F = P_3$ is the simplest meaningful case for which the complexity question remains open, and the hardness for this case likely extends to general $(F, H)$-Arrowing for nontrivial $F$. In this pursuit, we also gain insight into the complexity of a class of matching removal problems, since $(P_3, H)$-Arrowing is equivalent to $H$-free Matching Removal. We show that $(P_3, H)$-Arrowing is coNP-complete for all $2$-connected $H$ except when $H = K_3$, in which case the problem is in P. We introduce a new graph invariant to help us carefully combine graphs when constructing the gadgets for our reductions. Moreover, we show how $(P_3,H)$-Arrowing hardness results can be extended to other $(F,H)$-Arrowing problems. This allows for more intuitive and palatable hardness proofs instead of ad-hoc constructions of SAT gadgets, bringing us closer to categorizing the complexity of all $(F, H)$-Arrowing problems.