Counting Hamilton decompositions of oriented graphs

TL;DR

This paper estimates the number of Hamilton decompositions in large regular oriented graphs and provides a simpler proof for Kelly's conjecture, advancing understanding of Hamilton cycle structures.

Contribution

It quantifies the number of Hamilton decompositions in regular oriented graphs and offers a new, simpler proof for Kelly's conjecture.

Findings

Number of Hamilton decompositions is approximately $n^{(1-o(1))cn^2}$.

Established a simpler proof for the approximate Kelly's conjecture.

Extended results to large regular oriented graphs with degree proportional to $n$.

Abstract

A Hamilton cycle in a directed graph $G$ is a cycle that passes through every vertex of $G$. A Hamiltonian decomposition of $G$ is a partition of its edge set into disjoint Hamilton cycles. In the late $60$s Kelly conjectured that every regular tournament has a Hamilton decomposition. This conjecture was recently settled by K\"uhn and Osthus, who proved more generally that every $r$-regular $n$-vertex oriented graph $G$ (without antiparallel edges) with $r=cn$ for some fixed $c>3/8$ has a Hamiltonian decomposition, provided $n=n(c)$ is sufficiently large. In this paper we address the natural question of estimating the number of such decompositions of $G$ and show that this number is $n^{(1-o(1))cn^2}$. In addition, we also obtain a new and much simpler proof for the approximate version of Kelly's conjecture.