On kernels, defaults and even graphs

  • Yannis Dimopoulos
  • Vangelis Magirou
  • Christos H. Papadimitriou


Extensions in prerequisite‐free, disjunction‐free default theories have been shown to be in direct correspondence with kernels of directed graphs; hence default theories without odd cycles always have a “standard” kind of an extension. We show that, although all “standard” extensions can be enumerated explicitly, several other problems remain intractable for such theories: Telling whether a non‐standard extension exists, enumerating all extensions, and finding the minimal standard extension. We also present a new graph‐theoretic algorithm, based on vertex feedback sets, for enumerating all extensions of a general prerequisite‐free, disjunction‐free default theory (possibly with odd cycles). The algorithm empirically performs well for quite large theories.


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Copyright information

© Kluwer Academic Publishers 1997

Authors and Affiliations

  • Yannis Dimopoulos
    • 1
  • Vangelis Magirou
    • 2
  • Christos H. Papadimitriou
    • 3
  1. 1.Max‐Planck‐Institut für InformatikSaarbrückenGermany
  2. 2.Athens University of EconomicsAthensGreece
  3. 3.CS&EE DepartmentUniversity of California at San DiegoLa JollaUSA

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