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Witness of unsatisfiability for a random 3-satisfiability formula

Lu-Lu Wu and Hai-Jun Zhou

Mikko Alava

Erik Aurell

Pekka Orponen

  • State Key Laboratory for Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190, China

  • Department of Applied Physics, Aalto University, FI-00076 Aalto, Finland

  • ACCESS Linnaeus Center, KTH, Sweden, Department of Computational Biology, AlbaNova University Center, 10691 Stockholm, Sweden and Aalto University School of Science, FI-00076 Aalto, Finland

  • Department of Information and Computer Science, Aalto University, FI-00076 Aalto, Finland

Phys. Rev. E 87, 052807 – Published 20 May, 2013

DOI: https://doi.org/10.1103/PhysRevE.87.052807

Abstract

The random 3-satisfiability (3-SAT) problem is in the unsatisfiable (UNSAT) phase when the clause density α exceeds a critical value αs4.267. Rigorously proving the unsatisfiability of a given large 3-SAT instance is, however, extremely difficult. In this paper we apply the mean-field theory of statistical physics to the unsatisfiability problem, and show that a reduction to 3-XORSAT, which permits the construction of a specific type of UNSAT witnesses (Feige-Kim-Ofek witnesses), is possible when the clause density α>19. We then construct Feige-Kim-Ofek witnesses for single 3-SAT instances through a simple random sampling algorithm and a focused local search algorithm. The random sampling algorithm works only when α scales at least linearly with the variable number N, but the focused local search algorithm works for clause density α>cNb with b0.59 and prefactor c8. The exponent b can be further decreased by enlarging the single parameter S of the focused local search algorithm.

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