Pairs of SAT-assignments in random Boolean formulæ

  • Authors:
  • Hervé Daudé;Marc Mézard;Thierry Mora;Riccardo Zecchina

  • Affiliations:
  • LATP, UMR 6632 CNRS et Université de Provence, 13453 Marseille Cedex, France;LPTMS, UMR 8626 CNRS et Université Paris-Sud, 91405 Orsay Cedex, France;LPTMS, UMR 8626 CNRS et Université Paris-Sud, 91405 Orsay Cedex, France;Physics Department, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2008

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Abstract

We investigate geometrical properties of the random K-satisfiability problem using the notion of x-satisfiability: a formula is x-satisfiable is there exist two SAT-assignments differing in Nx variables. We show the existence of a sharp threshold for this property as a function of the clause density. For large enough K, we prove that there exists a region of clause density, below the satisfiability threshold, where the landscape of Hamming distances between SAT-assignments experiences a gap: pairs of SAT-assignments exist at small x, and around x=12, but they do not exist at intermediate values of x. This result is consistent with the clustering scenario which is at the heart of the recent heuristic analysis of satisfiability using statistical physics analysis (the cavity method), and its algorithmic counterpart (the survey propagation algorithm). Our method uses elementary probabilistic arguments (first and second moment methods), and might be useful in other problems of computational and physical interest where similar phenomena appear.