Space complexity of random formulae in resolution

  • Authors:

  • Eli Ben-Sasson;Nicola Galesi

  • Affiliations:

  • Institute of Computer Science, Hebrew University, Jerusalem, Israel;Universitat Politècnica de Catalaunya, Department Llenguatges i Sistemes Informátics, Barcelona 08034, Spain

  • Venue:
  • Random Structures & Algorithms
  • Year:
  • 2003

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Abstract

We study the space complexity of refuting unsatisfiable random k-CNFs in the Resolution proof system. We prove that for Δ ≥ 1 and any ε 0, with high probability a random k-CNF over n variables and Δn clauses requires resolution clause space of Ω(n/Δ1 + ε). For constant Δ, this gives us linear, optimal, lower bounds on the clause space. One consequence of this lower bound is the first lower bound for size of treelike resolution refutations of random 3-CNFs with clause density Δ √n. This bound is nearly tight. Specifically, we show that with high probability, a random 3-CNF with Δn clauses requires treelike refutation size of exp(Ω(n/Δ1 + ε). for any ε 0. Our space lower bound is the consequence of three main contributions: (1) We introduce a 2-player Matching Game on bipartite graphs G to prove that there are no perfect matchings in G. (2) We reduce lower bounds for the clause space of a formula F in Resolution to lower bounds for the complexity of the game played on the bipanite graph G(F) associated with F. (3) We prove that the complexity of the game is large whenever G is an expander graph. Finally, a simple probabilistic analysis shows that for a random formula F, with high probability G(F) is an expander. We also extend our result to the case of G-PHP, a generalization of the Pigeonhole principle based on bipartite graphs G.