Extended Horn sets in propositional logic

  • Authors:
  • V. Chandru;J. N. Hooker

  • Affiliations:
  • Purdue Univ., West Lafayette, IN;Carnegie-Mellon Univ., Pittsburgh, PA

  • Venue:
  • Journal of the ACM (JACM)
  • Year:
  • 1991

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Abstract

The class of Horn clause sets in propositional logic is extended to a larger class for which the satisfiability problem can still be solved by unit resolution in linear time. It is shown that to every arborescence there corresponds a family of extended Horn sets, where ordinary Horn sets correspond to stars with a root at the center. These results derive from a theorem of Chandresekaran that characterizes when an integer solution of a system of inequalities can be found by rounding a real solution in a certain way. A linear-time procedure is provided for identifying “hidden” extended Horn sets (extended Horn but for complementation of variables) that correspond to a specified arborescence. Finally, a way to interpret extended Horn sets in applications is suggested.