Abstract
For two positive integers k and \(\ell \), a \((k \times \ell )\)-spindle is the union of k pairwise internally vertex-disjoint directed paths with \(\ell \) arcs each between two vertices u and v. We are interested in the (parameterized) complexity of several problems consisting in deciding whether a given digraph contains a subdivision of a spindle, which generalize both the Maximum Flow and Longest Path problems. We obtain the following complexity dichotomy: for a fixed \(\ell \ge 1\), finding the largest k such that an input digraph G contains a subdivision of a \((k \times \ell )\)-spindle is polynomial-time solvable if \(\ell \le 3\), and NP-hard otherwise. We place special emphasis on finding spindles with exactly two paths and present FPT algorithms that are asymptotically optimal under the ETH. These algorithms are based on the technique of representative families in matroids, and use also color-coding as a subroutine. Finally, we study the case where the input graph is acyclic, and present several algorithmic and hardness results.
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Notes
The ETH states that there is no algorithm solving 3-SAT on a formula with n variables in time \(2^{o(n)}\).
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A conference version of this article appeared in the Proc. of the 13th Latin American Theoretical INformatics Symposium (LATIN), volume 10807 of LNCS, pages 66–79, Buenos Aires, Argentina, April 2018. This article is permanently available at https://arxiv.org/abs/1706.09066.
Work supported by Projects DEMOGRAPH (ANR-16-CE40-0028) and ESIGMA (ANR-17-CE23-0010), CNPq Grants 304576/2017-4 and 304478/2018-0, CNPq Universal Project 401519/2016-3, FUNCAP-PRONEM PNE-0112-00061.01.00/16 and CAPES-PRINT.
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Araújo, J., Campos, V.A., Maia, A.K. et al. On the Complexity of Finding Internally Vertex-Disjoint Long Directed Paths. Algorithmica 82, 1616–1639 (2020). https://doi.org/10.1007/s00453-019-00659-5
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DOI: https://doi.org/10.1007/s00453-019-00659-5
Keywords
- Digraph subdivision
- Spindle
- Parameterized complexity
- FPT algorithm
- Representative family
- Complexity dichotomy