Hamilton decompositions of regular expanders: A proof of Kelly’s conjecture for large tournaments

A long-standing conjecture of Kelly states that every regular tournament on nn vertices can be decomposed into (n−1)/2(n−1)/2 edge-disjoint Hamilton cycles. We prove this conjecture for large nn. In fact, we prove a far more general result, based on our recent concept of robust expansion and a new method for decomposing graphs. We show that every sufficiently large regular digraph GG on nn vertices whose degree is linear in nn and which is a robust outexpander has a decomposition into edge-disjoint Hamilton cycles. This enables us to obtain numerous further results, e.g. as a special case we confirm a conjecture of Erdős on packing Hamilton cycles in random tournaments. As corollaries to the main result, we also obtain several results on packing Hamilton cycles in undirected graphs, giving e.g. the best known result on a conjecture of Nash-Williams. We also apply our result to solve a problem on the domination ratio of the Asymmetric Travelling Salesman problem, which was raised e.g. by Glover and Punnen as well as Alon, Gutin and Krivelevich.