A half-integral Erd\H{o}s-P\'osa theorem for directed odd cycles

TL;DR

This paper establishes a half-integral Erdős-Pósa theorem for directed odd cycles, providing a function bounding the size of a hitting set or the number of cycles, along with polynomial-time algorithms for fixed parameters.

Contribution

It extends the half-integral Erdős-Pósa theorem to directed graphs and offers efficient algorithms for detecting or hitting directed odd cycles.

Findings

Existence of a function f(k) for directed odd cycles

Polynomial-time algorithm for fixed k to find cycles or hitting sets

Extension of Reed's undirected result to directed graphs

Abstract

We prove that there exists a function $f:\mathbb{N}\rightarrow \mathbb{R}$ such that every directed graph $G$ contains either $k$ directed odd cycles where every vertex of $G$ is contained in at most two of them, or a set of at most $f(k)$ vertices meeting all directed odd cycles. We also give a polynomial-time algorithm for fixed $k$ which outputs one of the two outcomes. Using this algorithmic result, we give a polynomial-time algorithm for fixed $k$ to decide whether such $k$ directed odd cycles exist, or there are no $k$ vertex-disjoint directed odd cycles. This extends the half-integral Erd\H{o}s-P\'osa theorem for undirected odd cycles by Reed [Combinatorica 1999] to directed graphs.