In this work we suggest a new model for generating random satisfiable k-CNF formulas. To generate such formulas. randomly permute all 2k (n/k) possible clauses over the variables x1,⋯,xn, and starting fürom the empty formula, go over the clauses one by one, including each new clause as you go along if, after its addition, the formula remains satisfiable. We study the evolution of this process, namely the distribution over formulas obtained after scanning through the first m clauses (in the random permutation's order). Random processes with conditioning on a certain property being respected are widely studied in the context of graph properties. This study was pioneered by Ruciski and Wormald in 1992 for graphs with a fixed degree sequence, and also by Erds, Suen and Winkler in 1995 for triangle-füree and bipartite graphs. Since then many other graph properties have been studied, such as planarity and H-füreeness. Thus our model is a natural extension of this approach to the satisfiability setting. Our main contribution is as follows. For m ≥ cn, c = c(k) a sufficiently large constant, we are able to characterize the structure of the solution space of a typical formula in this distribution. Specifically, we show that typically all satisfying assignments are essentially clustered in one cluster, and all but e-Ω(m/n)n of the variables take the same value in all satisfying assignments. We also describe a polynomial-time algorithm that finds w.h.p. a satisfying assignment for such formulas.
ASJC Scopus subject areas
- Theoretical Computer Science
- Statistics and Probability
- Computational Theory and Mathematics
- Applied Mathematics