A Dirac type result on Hamilton cycles in oriented graphs

TL;DR

This paper proves new minimum degree conditions under which large oriented graphs are guaranteed to contain Hamilton cycles, confirming conjectures and extending classical results in graph theory.

Contribution

It establishes a Dirac-type theorem for Hamiltonicity in oriented graphs with improved degree conditions, confirming a conjecture of H"aggkvist.

Findings

Large oriented graphs with minimum semi-degree above 3|G|/8 + α|G| are Hamiltonian.

A stronger condition involving the sum of degrees guarantees Hamiltonicity.

An Ore-type theorem for oriented graphs is also proved.

Abstract

We show that for each \alpha>0 every sufficiently large oriented graph G with \delta^+(G),\delta^-(G)\ge 3|G|/8+ \alpha |G| contains a Hamilton cycle. This gives an approximate solution to a problem of Thomassen. In fact, we prove the stronger result that G is still Hamiltonian if \delta(G)+\delta^+(G)+\delta^-(G)\geq 3|G|/2 + \alpha |G|. Up to the term \alpha |G| this confirms a conjecture of H\"aggkvist. We also prove an Ore-type theorem for oriented graphs.