Abstract
A digraph D = (V(D), A(D)) is called cycle-connected if for every pair of vertices \({u, v\in V(D)}\) there exists a cycle containing both u and v. Ádám (Acta Cybernet 14(1):1–12, 1999) proposed the question: Let D be a cycle-connected digraph. Does there exist a universal arc in D, i.e., an arc \({e\in A(D)}\) such that for every vertex \({w\in V(D)}\) there exists a cycle C in D containing both e and w?. Recently, Lutz Volkmann and Stefan Winzen have proved that this conjecture is true for multipartite tournaments. As an improvement of this result, we show in this note that every cycle-connected multipartite tournament with δ ≥ 2 contains at least two universal arcs.
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This work is supported by the National Natural Science Foundation of China (Grant No. 61070095, 11161035) and Ningxia Ziran (Grant No. NZ1153).
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Zou, Q., Li, G. & Gao, Y. Every Cycle-Connected Multipartite Tournament with δ ≥ 2 Contains At Least Two Universal Arcs. Graphs and Combinatorics 29, 1141–1149 (2013). https://doi.org/10.1007/s00373-012-1170-2
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DOI: https://doi.org/10.1007/s00373-012-1170-2