Short cycles in digraphs and the Caccetta-H\"{a}ggkvist conjecture

TL;DR

This paper advances understanding of the Caccetta-H"{a}ggkvist conjecture by proving it for specific classes of digraphs with large girth and no short even cycles, using novel graph constructions.

Contribution

It proves the conjectures for digraphs with girth at least their minimum out-degree and no short even cycles, extending known results through new graph product techniques.

Findings

Conjectures hold for digraphs with large girth and no short even cycles.

Uses properties of direct graph products and girth multiplication.

Provides conditions under which the conjectures are true.

Abstract

In the theory of digraphs, the study of cycles is a subject of great importance and has given birth to a number of deep questions such as the Behzad-Chartrand-Wall conjecture (1970) and its generalization, the Caccetta-H\"{a}ggkvist conjecture (1978). Despite a lot of interest and efforts, the progress on these remains slow and mostly restricted to the solution of some special cases. In this note, we prove these conjectures for digraphs with girth is at least as large as their minimum out-degree and without short even cycles. More generally, we prove that if a digraph has sufficiently large girth and does not contain closed walks of certain lengths, then the conjectures hold. The proof makes use of some of the known results on the Caccetta-H\"{a}ggkvist conjecture, properties of direct products of digraphs and a construction that multiplies the girth of a digraph.