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The number of out-pancyclic vertices in a strong tournament. (English) Zbl 1298.05144
Summary: An arc in a tournament $$T$$ with $$n\geq 3$$ vertices is called pancyclic, if it belongs to a cycle of length $$l$$ for all $$3\leq l\leq n$$. We call a vertex $$u$$ of $$T$$ an out-pancyclic vertex of $$T$$, if each out-arc of $$u$$ is pancyclic in $$T$$. T. Yao et al. [Discrete Appl. Math. 99, No. 1–3, 245–249 (2000; Zbl 0939.05045)] proved that every strong tournament contains an out-pancyclic vertex. For strong tournaments with minimum out-degree 1, Yao et al. [loc. cit.] found an infinite class of strong tournaments, each of which contains exactly one out-pancyclic vertex. In this paper, we prove that every strong tournament with minimum out-degree at least 2 contains three out-pancyclic vertices. Our result is best possible since there is an infinite family of strong tournaments with minimum degree at least 2 and no more than 3 out-pancyclic vertices.

##### MSC:
 05C20 Directed graphs (digraphs), tournaments 05C38 Paths and cycles
##### Keywords:
tournaments; out-arcs; pancyclicity
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##### References:
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