Width-parameterized SAT: Time-Space Tradeoffs
Shiteng Chen, Tiancheng Lou, Periklis A. Papakonstantinou, Bangsheng Tang · arXiv (Cornell University) · 2011
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 $(O^*(2^{2tw(ϕ)}), O^*(2^{tw(ϕ)}))$ and another that runs in time-space $(O^*(3^{tw(ϕ)\log{|ϕ|}}),|ϕ|^{O(1)})$, where $tw(ϕ)$ is the tree-width of a formula $ϕ$ with $|ϕ|$ 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 $2^{tw(ϕ)\mathbf{o(\log{|ϕ|})}}$ necessarily blows up the space to exponential in $tw(ϕ)$. 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 $(O^*(3^{1.441(1-ε)tw(ϕ)\log{|ϕ|}}), O^*(2^{2εtw(ϕ)}))$, for every $0