Clique-width minimization is NP-hard

  • Authors:
  • Michael R. Fellows;Frances A. Rosamond;Udi Rotics;Stefan Szeider

  • Affiliations:
  • University of Newcastle, Callaghan, NSW, Australia;University of Newcastle, Callaghan, NSW, Australia;Netanya Academic College, Netanya, Israel;Durham University, Durham, England, UK

  • Venue:
  • Proceedings of the thirty-eighth annual ACM symposium on Theory of computing
  • Year:
  • 2006

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Abstract

Clique-width is a graph parameter that measures in a certain sense the complexity of a graph. Hard graph problems (e.g., problems expressible in Monadic Second Order Logic with second-order quantification on vertex sets, that includes NP-hard problems) can be solved efficiently for graphs of small clique-width. It is widely believed that determining the clique-width of a graph is NP-hard; in spite of considerable efforts, no NP-hardness proof has been found so far. We give the first hardness proof. We show that the clique-width of a given graph cannot be absolutely approximated in polynomial time unless P=NP. We also show that, given a graph G and an integer k, deciding whether the clique-width of G is at most k is NPhy complete. This solves a problem that has been open since the introduction of clique-width in the early 1990s.