Proof of the Caccetta–Häggkvist conjecture for digraphs with small independence number

Abstract

For a digraph G and v∈V(G), let δ+(v) be the number of out-neighbors of v in G. The Caccetta–Häggkvist conjecture states that for all k≥1, if G is a digraph with n=|V(G)| such that δ+(v)≥n∕k for all v∈V(G), then G contains a directed cycle of length at most k. In [2], N. Lichiardopol proved that this conjecture is true for digraphs with independence number equal to two. In this paper, we generalize that result, proving that the conjecture is true for digraphs with independence number at most (k+1)∕2.