Width-parameterized SAT: Time-Space Tradeoffs

TL;DR

This paper introduces new algorithms for SAT parameterized by width measures like tree-width, achieving novel time-space tradeoffs and exploring the limits of such techniques, with implications for complexity theory.

Contribution

The paper presents two simple algorithms with specific time-space bounds for width-parameterized SAT and introduces a novel method to trade constants in the exponents, expanding the algorithmic toolkit.

Findings

Algorithms with specific time-space bounds based on tree-width.

A new technique to trade constants in the exponents of time and space.

Lower bounds and complexity separations for width-parameterized SAT.

Abstract

Width parameterizations of SAT, such as tree-width and path-width, enable the study of computationally more tractable and practical SAT instances. We give two simple algorithms. One that runs simultaneously in time-space $(O^*(2^{2tw(\phi)}), O^*(2^{tw(\phi)}))$ and another that runs in time-space $(O^*(3^{tw(\phi)\log{|\phi|}}),|\phi|^{O(1)})$, where $tw(\phi)$ is the tree-width of a formula $\phi$ with $|\phi|$ many clauses and variables. This partially answers the question of Alekhnovitch and Razborov, who also gave algorithms exponential both in time and space, and asked whether the space can be made smaller. We conjecture that every algorithm for this problem that runs in time $2^{tw(\phi)\mathbf{o(\log{|\phi|})}}$ necessarily blows up the space to exponential in $tw(\phi)$. We introduce a novel way to combine the two simple algorithms that allows us to trade \emph{constant} factors in the exponents between running time and space. Our technique gives rise to a family of algorithms controlled by two parameters. By fixing one parameter we obtain an algorithm that runs in time-space $(O^*(3^{1.441(1-\epsilon)tw(\phi)\log{|\phi|}}), O^*(2^{2\epsilon tw(\phi)}))$, for every $0<\epsilon<1$. We systematically study the limitations of this technique, and show that these algorithmic results are the best achievable using this technique. We also study further the computational complexity of width parameterizations of SAT. We prove non-sparsification lower bounds for formulas of path-width $\omega(\log|\phi|)$, and a separation between the complexity of path-width and tree-width parametrized SAT modulo plausible complexity assumptions.