Conflict-Free Cuts in Planar and 3-Degenerate Graphs with 1-Regular Conflicts

TL;DR

This paper investigates the complexity of finding conflict-free cuts in planar and 3-degenerate graphs with 1-regular conflict graphs, providing complete classifications and new constructions.

Contribution

It fully characterizes the computational complexity of conflict-free cuts in these graph classes and resolves open questions from prior research.

Findings

Conflict-free cuts always exist in planar 4-regular graphs except the octahedron.

Deciding conflict-free cuts is NP-complete for planar graphs with maximum degree 5.

The problem is NP-complete for 3-degenerate graphs with maximum degree 5.

Abstract

A conflict-free cut $F$ on a simple connected graph $G = (V, E)$ is defined as a set of edges $F \subseteq E$ such that $G-F$ is disconnected, and no two edges in $F$ are conflicting. The notion of conflicting edges is represented using an associated conflict graph $\widehat{G} = (\widehat{V}, \widehat{E})$ where $\widehat{V} = E$. Deciding if a given planar graph $G$, with an associated conflict graph $\widehat{G}$, has a conflict-free cut is known to be NP-complete, when $G$ has maximum degree four and $\widehat{G}$ is a line graph of $G$ [Bonsma, JGT 2009]. In this paper, we prove the following for the case when $\widehat{G}$ is 1-regular. * We completely resolve the complexity of the decision problem when $G$ is planar. Towards this end, we show that (a) there always exists a conflict-free cut when the graph is planar and 4-regular unless it is the octahedron graph and (b) the decision problem is NP-complete, even in the case when $G$ is planar with maximum degree 5. * We also show that the decision problem is NP-complete when $G$ is a 3-degenerate graph with maximum degree 5. This completely resolves the complexity status of the problem when $G$ is 3-degenerate. * We construct families of graphs with 1-regular conflict graphs that do not have a conflict-free cut. Our results answer the questions posed in [Rauch, Rautenbach and Souza, IPL 2025].