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Packing Arc-Disjoint Cycles in Tournaments

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Abstract

A tournament is a directed graph in which there is a single arc between every pair of distinct vertices. Given a tournament T on n vertices, we explore the classical and parameterized complexity of the problems of determining if T has a cycle packing (a set of pairwise arc-disjoint cycles) of size k and a triangle packing (a set of pairwise arc-disjoint triangles) of size k. We refer to these problems as Arc-disjoint Cycles in Tournaments (ACT) and Arc-disjoint Triangles in Tournaments (ATT), respectively. Although the maximization version of ACT can be seen as the dual of the well-studied problem of finding a minimum feedback arc set (a set of arcs whose deletion results in an acyclic graph) in tournaments, surprisingly no algorithmic results seem to exist for ACT. We first show that ACT and ATT are both NP-complete. Then, we show that the problem of determining if a tournament has a cycle packing and a feedback arc set of the same size is NP-complete. Next, we prove that ACT is fixed-parameter tractable via a \(2^{\mathcal {O}(k \log k)} n^{\mathcal {O}(1)}\)-time algorithm and admits a kernel with \(\mathcal {O}(k)\) vertices. Then, we show that ATT too has a kernel with \(\mathcal {O}(k)\) vertices and can be solved in \(2^{\mathcal {O}(k)} n^{\mathcal {O}(1)}\) time. Afterwards, we describe polynomial-time algorithms for ACT and ATT when the input tournament has a feedback arc set that is a matching. We also prove that ACT and ATT cannot be solved in \(2^{o(\sqrt{n})} n^{\mathcal {O}(1)}\) time under the exponential-time hypothesis.

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Notes

  1. The authors would like to thank F. Havet for pointing out that Lemma 13 was a consequence of a result by Chudnovsky et al. [15], as well for an improvement of the constant in Theorem 14.

  2. Lemma 15 is Lemma 3.9 of [10] that has been rephrased to avoid the use of several definitions and terminology introduced in [10].

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Acknowledgements

Saket Saurabh is supported by funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 819416) and also acknowledges the support of Swarnajayanti Fellowship Grant DST/SJF/MSA-01/2017-18.

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Authors and Affiliations

  1. LIRMM, CNRS, Université de Montpellier, Montpellier, France

    Stéphane Bessy, Marin Bougeret & Jocelyn Thiebaut

  2. Indian Institute of Technology Palakkad, Palakkad, India

    R. Krithika

  3. The Institute of Mathematical Sciences, HBNI, Chennai, India

    Abhishek Sahu & Saket Saurabh

  4. Ben-Gurion University, Beersheba, Israel

    Meirav Zehavi

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  1. Stéphane Bessy
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Correspondence to R. Krithika.

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This paper is based on the two independent manuscripts [9] and [36]. A preliminary version of this work appears in the Proceedings of the 44th International Symposium on Mathematical Foundations of Computer Science (MFCS 2019).

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Bessy, S., Bougeret, M., Krithika, R. et al. Packing Arc-Disjoint Cycles in Tournaments. Algorithmica 83, 1393–1420 (2021). https://doi.org/10.1007/s00453-020-00788-2

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