Induced paths in graphs without anticomplete cycles

TL;DR

This paper proves a conjecture that $s\mathcal{O}$-free graphs have polynomially many induced paths, enabling polynomial-time testing for such graphs, and advances understanding of their structure.

Contribution

It confirms Le's conjecture by showing $s\mathcal{O}$-free graphs have polynomially bounded induced paths, and provides a polynomial-time algorithm for testing this property.

Findings

Proved that $s\mathcal{O}$-free graphs have polynomially many induced paths.

Established a polynomial-time algorithm to test if a graph is $s\mathcal{O}$-free.

Connected structural properties of $s\mathcal{O}$-free graphs with cycle restrictions.

Abstract

Let us say a graph is $s\mathcal{O}$-free, where $s\ge 1$ is an integer, if there do not exist $s$ cycles of the graph that are pairwise vertex-disjoint and have no edges joining them. The structure of such graphs, even when $s=2$, is not well understood. For instance, until now we did not know how to test whether a graph is $2\mathcal{O}$-free in polynomial time; and there was an open conjecture, due to Ngoc Khang Le, that $2\mathcal{O}$-free graphs have only a polynomial number of induced paths. In this paper we prove Le's conjecture; indeed, we will show that for all $s\ge 1$, there exists $c>0$ such that every $s\mathcal{O}$-free graph $G$ has at most $|G|^c$ induced paths. This provides a poly-time algorithm to test if a graph is $s\mathcal{O}$-free, for all fixed $s$. The proof has three parts. First, there is a short and beautiful proof, due to Le, that reduces the question to proving the same thing for graphs with no cycles of length four. Second, there is a recent result of Bonamy, Bonnet, D\'epr\'es, Esperet, Geniet, Hilaire, Thomass\'e and Wesolek, that in every $s\mathcal{O}$-free graph $G$ with no cycle of length four, there is a set of vertices that intersects every cycle, with size logarithmic in $|G|$. And third, there is an argument that uses the result of Bonamy et al. to deduce the theorem. The last is the main content of this paper.