Time-Space Tradeoffs for Counting NP Solutions Modulo Integers

  • Authors:
  • R. Ryan Williams

  • Affiliations:
  • Computer Science Department, Carnegie Mellon University, Pittsburgh, USA 15213

  • Venue:
  • Computational Complexity
  • Year:
  • 2008

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Abstract

We prove the first time-space tradeoffs for counting the number of solutions to an NP problem modulo small integers, and also improve upon known time-space tradeoffs for Sat. Let m 0 be an integer, and define MOD m -Sat to be the problem of determining if a given Boolean formula has exactly km satisfying assignments, for some integer k. We show for all primes p except for possibly one of them, and for all c d 0 such that MOD p -Sat is not solvable in n c time and n d space by general algorithms. That is, there is at most one prime p that does not satisfy the tradeoff.We prove that the same limitation holds for Sat and MOD 6-Sat, as well as MOD m -Sat for any composite m that is not a prime power. Our main tool is a general method for rapidly simulating deterministic computations with restricted space, by counting the number of solutions to NP predicates modulo integers. The simulation converts an ordinary algorithm into a "canonical" one that consumes roughly the same amount of time and space, yet canonical algorithms have nice properties suitable for counting.