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Journal Articles Journal of Graph Theory Year : 2020

Out‐colourings of digraphs


We study vertex colourings of digraphs so that no out-neighbourhood is monochromatic and call such a colouring an out-colouring. The problem of deciding whether a given digraph has an out-colouring with only two colours (called a 2-out-colouring) is NP-complete. We show that for every choice of positive integers r, k there exists a k-strong bipartite tournament which needs at least r colours in every out-colouring. Our main results are on tournaments and semicomplete digraphs. We prove that, except for the Paley tournament P7, every strong semicomplete digraph of minimum out-degree at least 3 has a 2-out-colouring. Furthermore, we show that every semi- complete digraph on at least 7 vertices has a 2-out-colouring if and only if it has a balanced such colouring, that is, the difference between the number of vertices that receive colour 1 and colour 2 is at most one. In the second half of the paper we consider the generalization of 2-out-colourings to vertex partitions (V1,V2) of a digraph D so that each of the three digraphs induced by re- spectively, the vertices of V1, the vertices of V2 and all arcs between V1 and V2 have minimum out-degree k for a prescribed integer k ≥ 1. Using probabilistic arguments we prove that there exists an absolute positive constant c so that every semicomplete digraph of minimum out-degree √ at least 2k + c k has such a partition. This is tight up to the value of c.

Dates and versions

lirmm-03271566 , version 1 (26-06-2021)



Noga Alon, Jørgen Bang‐jensen, Stéphane Bessy. Out‐colourings of digraphs. Journal of Graph Theory, 2020, 93 (1), pp.88-112. ⟨10.1002/jgt.22476⟩. ⟨lirmm-03271566⟩
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