Partitioning graphs into connected parts

  • Authors:
  • Pim van t Hof;Daniël Paulusma;Gerhard J. Woeginger

  • Affiliations:
  • Department of Computer Science, Durham University, Science Laboratories, South Road, Durham DH1 3LE, England, United Kingdom;Department of Computer Science, Durham University, Science Laboratories, South Road, Durham DH1 3LE, England, United Kingdom;Department of Mathematics and Computer Science, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2009

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Abstract

The 2-Disjoint Connected Subgraphs problem asks if a given graph has two vertex-disjoint connected subgraphs containing prespecified sets of vertices. We show that this problem is NP-complete even if one of the sets has cardinality 2. The Longest Path Contractibility problem asks for the largest integer @? for which an input graph can be contracted to the path P"@? on @? vertices. We show that the computational complexity of the Longest Path Contractibility problem restricted to P"@?-free graphs jumps from being polynomially solvable to being NP-hard at @?=6, while this jump occurs at @?=5 for the 2-Disjoint Connected Subgraphs problem. We also present an exact algorithm that solves the 2-Disjoint Connected Subgraphs problem faster than O^*(2^n) for any n-vertex P"@?-free graph. For @?=6, its running time is O^*(1.5790^n). We modify this algorithm to solve the Longest Path Contractibility problem for P"6-free graphs in O^*(1.5790^n) time.