On monochromatic paths in edge-coloured digraphs

AbstractLet G be a directed graph whose edges are coloured with two colours. Call a set S of vertices of Gindependent if no two vertices of S are connected by a monochromatic directed path. We prove that if G contains no monochromatic infinite outward path, then there is an independent set S of vertices of G such that, for every vertex x not in S, there is a monochromatic directed path from x to a vertex of S. In the event that G is infinite, the proof uses Zorn's lemma. The last part of the paper is concerned with the case when G is a tournament