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Mining circuit lower bound proofs for metaalgorithms
, 2013
"... We show that circuit lower bound proofs based on the method of random restrictions yield nontrivial compression algorithms for “easy ” Boolean functions from the corresponding circuit classes. The compression problem is defined as follows: given the truth table of an nvariate Boolean function f co ..."
Abstract

Cited by 6 (0 self)
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We show that circuit lower bound proofs based on the method of random restrictions yield nontrivial compression algorithms for “easy ” Boolean functions from the corresponding circuit classes. The compression problem is defined as follows: given the truth table of an nvariate Boolean function f computable by some unknown small circuit from a known class of circuits, find in deterministic time poly(2n) a circuit C (no restriction on the type of C) computing f so that the size of C is less than the trivial circuit size 2n/n. We get nontrivial compression for functions computable by AC0 circuits, (de Morgan) formulas, and (readonce) branching programs of the size for which the lower bounds for the corresponding circuit class are known. These compression algorithms rely on the structural characterizations of “easy ” functions, which are useful both for proving circuit lower bounds and for designing “metaalgorithms” (such as CircuitSAT). For (de Morgan) formulas, such structural characterization is provided by the “shrinkage under random restrictions ” results [Sub61, H̊as98], strengthened to the “highprobability ” version by [San10, IMZ12, KR13]. We give a new, simple proof of the “highprobability ” version of the shrinkage result for (de Morgan) formulas, with improved parameters. We use this shrinkage result to get both compression and #SAT algorithms for (de Morgan) formulas of size about n2. We also use this shrinkage result to get an alternative proof of the recent result by Komargodski and Raz [KR13] of the averagecase lower bound against small (de Morgan) formulas. Finally, we show that the existence of any nontrivial compression algorithm for a circuit class C ⊆ P/poly would imply the circuit lower bound NEXP 6 ⊆ C; a similar implication is independently proved also by Williams [Wil13]. This complements Williams’s result [Wil10] that any nontrivial CircuitSAT algorithm for a circuit class C would imply a superpolynomial lower bound against C for a language in NEXP.
Local reductions
, 2013
"... We reduce nondeterministic time T ≥ 2 n to a 3SAT instance φ of size φ  = T ·log O(1) T such that there is an explicit circuit C that on input an index i of logφ bits outputs the ith clause, and each output bit of C depends on O(1) inputs bits. The previous best result was C in NC 1. Even in th ..."
Abstract

Cited by 3 (1 self)
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We reduce nondeterministic time T ≥ 2 n to a 3SAT instance φ of size φ  = T ·log O(1) T such that there is an explicit circuit C that on input an index i of logφ bits outputs the ith clause, and each output bit of C depends on O(1) inputs bits. The previous best result was C in NC 1. Even in the simpler setting of φ  = poly(T) the previous best result was C in AC 0. More generally, for any time T ≥ n and parameter r ≤ n we obtain log 2φ  = max(logT,n/r)+O(logn)+O(loglogT) and each output bit of C is a decision tree of depth O(logr). As an application, we simplify the proof of Williams ’ ACC 0 lower bound, and tighten his connection between satisfiability algorithms and lower bounds.