On the critical exponents of random k-SAT

  • Authors:
  • David B. Wilson

  • Affiliations:
  • One Microsoft Way, Redmond, WA

  • Venue:
  • Random Structures & Algorithms
  • Year:
  • 2002

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Abstract

There has been much recent interest in the satisfiability of random Boolean formulas. A random k-SAT formula is the conjunction of m random clauses, each of which is the disjunction of k literals (a variable or its negation). It is known that when the number of variables n is large, there is a sharp transition from satisfiability to unsatisfiability; in the case of 2-SAT this happens when m/n → 1, for 3-SAT the critical ratio is thought to be m/n ≈ 4.2. The sharpness of this transition is characterized by a critical exponent, sometimes called v = vk (the smaller the value of v the sharper the transition). Experiments have suggested that v3 = 1.5 ± 0.1. v4 = 1.25 ± 0.05, v5 = 1.1 ± 0.05, v6 = 1.05 ± 0.05, and heuristics have suggested that vk → 1 as k → ∞. We give here a simple proof that each of these exponents is at least 2 (provided the exponent is well defined). This result holds for each of the three standard ensembles of random k-SAT formulas: m clauses selected uniformly at random without replacement, m clauses selected uniformly at random with replacement, and each clause selected with probability p independent of the other clauses. We also obtain similar results for q-colorability and the appearance of a q-core in a random graph.