Conflict packing yields linear vertex-kernels for k-FAST, k-dense RTI and a related problem

  • Authors:
  • Christophe Paul;Anthony Perez;Stéphan Thomassé

  • Affiliations:
  • LIRMM, Université Montpellier2, CNRS, France;LIRMM, Université Montpellier2, CNRS, France;LIRMM, Université Montpellier2, CNRS, France

  • Venue:
  • MFCS'11 Proceedings of the 36th international conference on Mathematical foundations of computer science
  • Year:
  • 2011

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Abstract

We develop a technique that we call Conflict Packing in the context of kernelization [7]. We illustrate this technique on several well-studied problems: FEEDBACK ARC SET IN TOURNAMENTS, DENSE ROOTED TRIPLET INCONSISTENCY and BETWEENNESS IN TOURNAMENTS. For the former, one is given a tournament T = (V, A) and seeks a set of at most k arcs whose reversal in T results in an acyclic tournament. While a linear vertex-kernel is already known for this problem [6], using the Conflict Packing allows us to find a so-called safe partition, the central tool of the kernelization algorithm in [6], with simpler arguments. Regarding the DENSE ROOTED TRIPLET INCONSISTENCY problem, one is given a set of vertices V and a dense collection R of rooted binary trees over three vertices of V and seeks a rooted tree over V containing all but at most k triplets from R. Using again the Conflict Packing, we prove that the DENSE ROOTED TRIPLET INCONSISTENCY problem admits a linear vertex-kernel. This result improves the best known bound of O(k2) vertices for this problem [16]. Finally, we use this technique to obtain a linear vertex-kernel for BETWEENNESS IN TOURNAMENTS, where one is given a set of vertices V and a dense collection R of betweenness triplets and seeks an ordering containing all but at most k triplets from R. To the best of our knowledge this result constitutes the first polynomial kernel for the problem.