Aboulker et al.'s conjecture for out-stars and the directed triangle

Let \accentsetK2\accentset{\leftrightarrow}{K}_2 be the complete digraph on two vertices, let S2+S_2^+ be the orientation of a two-leaf star with all arcs directed outwards, and let C3\vec{C}_3 be the directed cycle on three vertices. For a family of forbidden induced digraphs, write Forbind()\operatorname{Forb}_{\operatorname{ind}}(\cdot) for the corresponding class and χ\vec{\chi} for dichromatic number. Aboulker et al.'s conjecture. The two forbidden-subdigraph classes satisfy

χ(Forbind(\accentsetK2,S2+,C3))=χ(Forbind(\accentsetK2,S2,C3))=2.\vec{\chi}(\operatorname{Forb}_{\operatorname{ind}}(\accentset{\leftrightarrow}{K}_2,S_2^+,\vec{C}_3))=\vec{\chi}(\operatorname{Forb}_{\operatorname{ind}}(\accentset{\leftrightarrow}{K}_2,S_2^-,\vec{C}_3))=2.

The source states that this was open for the two orientations of the three-vertex star, while the paper's conclusion says that it resolves this conjecture; hence its status is recorded as solved.

Sources & referencesView supporting material

Primary source

Raphael Steiner, “On coloring digraphs with forbidden induced subgraphs”, arXiv:2103.04191 (2021).

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