Disjoint cycles intersecting a set of vertices

  • Authors:
  • M. Pontecorvi;P. Wollan

  • Affiliations:
  • Department of Computer Science, University of Texas at Austin, TX, United States;Department of Computer Science, University of Rome, "La Sapienza" Via Salaria 113, Rome, 00198, Italy

  • Venue:
  • Journal of Combinatorial Theory Series B
  • Year:
  • 2012

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Abstract

A classic theorem of Erdos and Posa states that there exists a constant c such that for all positive integers k and graphs G, either G contains k vertex disjoint cycles, or there exists a subset of at most cklogk vertices intersecting every cycle of G. We consider the following generalization of the problem: fix a subset S of vertices of G. An S-cycle is a cycle containing at least one vertex of S. We show that again there exists a constant c^' such that G either contains k disjoint S-cycles, or there exists a set of at most c^'klogk vertices intersecting every S-cycle. The proof yields an algorithm for finding either the disjoint S-cycles or the set of vertices intersecting every S-cycle. An immediate consequence is an O(logn)-approximation algorithm for finding disjoint S-cycles.