Geodesic
Monochromatic paths and monochromatic sets of arcs in quasi-transitive digraphs
Discussiones Mathematicae. Graph Theory, Tome 30 (2010) no. 4, pp. 545-553.

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Let D be a digraph, V(D) and A(D) will denote the sets of vertices and arcs of D, respectively. We call the digraph D an m-coloured digraph if each arc of D is coloured by an element of 1,2,...,m where m ≥ 1. A directed path is called monochromatic if all of its arcs are coloured alike. A set N of vertices of D is called a kernel by monochromatic paths if there is no monochromatic path between two vertices of N and if for every vertex v not in N there is a monochromatic path from v to some vertex in N. A digraph D is called a quasi-transitive digraph if (u,v) ∈ A(D) and (v,w) ∈ A(D) implies (u,w) ∈ A(D) or (w,u) ∈ A(D). We prove that if D is an m-coloured quasi-transitive digraph such that for every vertex u of D the set of arcs that have u as initial end point is monochromatic and D contains no C₃ (the 3-coloured directed cycle of length 3), then D has a kernel by monochromatic paths.
Keywords: m-coloured quasi-transitive digraph, kernel by monochromatic paths 
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Galeana-Sánchez, Hortensia; Rojas-Monroy, R.; Zavala, B. Monochromatic paths and monochromatic sets of arcs in quasi-transitive digraphs. Discussiones Mathematicae. Graph Theory, Tome 30 (2010) no. 4, pp. 545-553. http://geodesic.mathdoc.fr/item/DMGT_2010_30_4_a1/

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