Space characterizations of complexity measures and size-space trade-offs in propositional proof systems

Abstract

We identify two new clusters of proof complexity measures equal up to polynomial and log⁡n factors. The first cluster contains the logarithm of tree-like resolution size, regularized clause and monomial space, and clause space, ordinary and regularized, in regular and tree-like resolution. Consequently, separating clause or monomial space from the logarithm of tree-like resolution size is equivalent to showing strong trade-offs between clause space and length, and equivalent to showing super-critical trade-offs between clause space and depth. The second cluster contains width, Σ2 space (a generalization of clause space to depth 2 Frege systems), ordinary and regularized, and the logarithm of tree-like R(log⁡) size. As an application, we improve a known size-space trade-off for polynomial calculus with resolution. We further show a quadratic lower bound on tree-like resolution size for formulas refutable in clause space 4, and introduce a measure intermediate between depth and the logarithm of tree-like resolution size.