Variable independence and resolution paths for quantified boolean formulas

  • Authors:
  • Allen Van Gelder

  • Affiliations:
  • University of California, Santa Cruz

  • Venue:
  • CP'11 Proceedings of the 17th international conference on Principles and practice of constraint programming
  • Year:
  • 2011

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Abstract

Variable independence in quantified boolean formulas (QBFs) informally means that the quantifier structure of the formula can be rearranged so that two variables reverse their outer-inner relationship without changing the value of the QBF. Samer and Szeider introduced the standard dependency scheme and the triangle dependency scheme to safely over-approximate the set of variable pairs for which an outer-inner reversal might be unsound (JAR 2009). This paper introduces resolution paths and defines the resolution-path dependency relation. The resolution-path relation is shown to be the root (smallest) of a lattice of dependency relations that includes quadrangle dependencies, triangle dependencies, strict standard dependencies, and standard dependencies. Soundness is proved for resolution-path dependencies, thus proving soundness for all the descendants in the lattice. It is shown that the biconnected components (BCCs) and block trees of a certain clause-literal graph provide the key to computing dependency pairs efficiently for quadrangle dependencies. Preliminary empirical results on the 568 QBFEVAL-10 benchmarks show that in the outermost two quantifier blocks quadrangle dependency relations are smaller than standard dependency relations by widely varying factors.