New results on openly disjoint cycles in regular digraphs

Abstract

Given a digraph \(D\), let \(c(D)\) be the largest size of a collection of openly disjoint directed cycles through a common vertex, and let \(c_r\) be the minimum of \(c(D)\) over all \(r\)-regular digraphs \(D\). In 2005, Seymour asked whether every \(r\)-regular digraph satisfies \(c(D)=r\). Mader answered Seymour's problem negatively for every \(r\ge 8\), but conjectured that nevertheless \(\lim_{r\to\infty}c_r=\infty\). Recently, Steiner confirmed this conjecture by proving \[ \left\lceil \frac{3r}{22}\right\rceil \le c_r \le 7\left\lceil \frac{r}{8}\right\rceil. \] In this paper, we first answer Seymour's problem negatively for $r\in \{6,7\}$, and then improve Steiner's bound to \[ \left\lceil \frac{r}{6}\right\rceil \le c_r \le 5\left\lceil \frac{r}{6}\right\rceil.\]