TY - JOUR
TI - Width-parameterized SAT: time-space tradeoffs,
DO - https://doi.org/doi:10.7282/T3N87CM1
AU - Allender, Eric
AU - Chen, Shiteng
AU - Lou, Tiancheng
AU - Papakonstantinou, Periklis
AU - Tang, Bangsheng
PY - 2014
T2 - Theory of Computing
VL - 10
IS - 12
SP - 297
EP - 339
AB - Alekhnovich and Razborov (2002) presented an algorithm that solves SAT on instances ϕ of size n and tree-width TW(ϕ), using time and space bounded by 2O(TW(ϕ))nO(1). Although several follow-up works appeared over the last decade, the first open question of Alekhnovich and Razborov remained essentially unresolved: Can one check satisfiability of formulas with small tree-width in polynomial space and time as above? We essentially resolve this question, by (1) giving a polynomial space algorithm with a slightly worse run-time, (2) providing a complexity-theoretic characterization of bounded tree-width SAT, which strongly suggests that no polynomial-space algorithm can run significantly faster, and (3) presenting a spectrum of algorithms trading off time for space, between our PSPACE algorithm and the fastest known algorithm.
First, we give a simple algorithm that runs in polynomial space and achieves run-time 3TW(ϕ)lognnO(1), which approaches the run-time of Alekhnovich and Razborov (2002), but has an additional log n factor in the exponent. Then, we conjecture that this annoying log n factor is in general unavoidable.
Our negative results show our conjecture true if one believes a well-known complexity assumption, which is the SC ≠ NC conjecture and its scaled variants. Technically, we base our result on the following lemma. For arbitrary k, SAT of tree-width logkn is complete for the class of problems computed by circuits of logarithmic depth, semi-unbounded fan-in and size 2O(logkn) (SAC1 when k=1). Problems in this class can be solved simultaneously in time-space (2O(logk+1n),O(logk+1n)), and also in (2O(logkn), 2O(logkn)). Then, we show that our conjecture (for SAT instances with poly-log tree-width) is equivalent to the question of whether the small-space simulation of semi-unbounded circuit classes can be sped up without incurring a large space penalty. This is a recasting of the conjecture that SAC1 (and even its subclass NL) is not contained in SC.
Although we cannot hope for an improvement asymptotically in the exponent of time and space, we introduce a new algorithmic technique which trades constants in the exponents: for each ε with 0<ε<1, we give an algorithm in time-space
(31.441(1−ε)TW(ϕ)log|ϕ||ϕ|O(1),22εTW(ϕ)|ϕ|O(1)).
We systematically study the limitations of our technique for trading off time and space, and we show that our bounds are the best achievable using this technique.
KW - Complexity theory
KW - Algorithms
KW - Complexity classes
KW - Circuit complexity
KW - Parametrized complexity
KW - Nondeterminism
KW - CNF-DNF formulas
KW - Boolean formulas
KW - Completeness
KW - Time-space tradeoff
KW - Treewidth (Graph theory)
KW - Pathwidth (Graph theory)
KW - Computational complexity
LA - English
ER -