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29 p. : il.The class of unichord-free graphs was recently investigated
in the context of vertex-colouring [J. Graph Theory 63 (2010) 31{67],
edge-colouring [Theoret. Comput. Sci. 411 (2010) 1221{1234] and total-
colouring [Discrete Appl. Math. 159 (2011) 1851{1864]. Unichord-free
graphs proved to have a rich structure that can be used to obtain in-
teresting results with respect to the study of the complexity of colour-
ing problems. In particular, several surprising complexity dichotomies of
colouring problems are found in subclasses of unichord-free graphs. In
the present work, we investigate clique-colouring and biclique-colouring
problems restricted to unichord-free graphs. We show that the clique-
chromatic number of a unichord-free graph is at most 3, and that the
2-clique-colourable unichord-free graphs are precisely those that are per-
fect. We prove that the biclique-chromatic number of a unichord-free
graph is at most its clique-number. We describe an O(nm)-time algo-
rithm that returns an optimal clique-colouring, but the complexity to
optimal biclique-colour a unichord-free graph is not classi ed yet. Nev-
ertheless, we describe an O(n2)-time algorithm that returns an optimal
biclique-colouring in a subclass of unichord-free graphs called cactus.
Keywords: unichord-free, decomposition, hypergraphs, Petersen graph,
Heawood graph, clique-colouring, biclique-colouring, cactus
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