Limits on the computational power of random strings

  • Authors:
  • Eric Allender;Luke Friedman;William Gasarch

  • Affiliations:
  • Department of Computer Science, Rutgers University, Piscataway, NJ;Department of Computer Science, Rutgers University, Piscataway, NJ;Dept. of Computer Science, University of Maryland, College Park, MD

  • Venue:
  • ICALP'11 Proceedings of the 38th international colloquim conference on Automata, languages and programming - Volume Part I
  • Year:
  • 2011

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Abstract

How powerful is the set of random strings? What can one say about a set A that is efficiently reducible to R, the set of Kolmogorov-random strings? We present the first upper bound on the class of computable sets in PR and NPR. The two mostwidely-studied notions ofKolmogorov complexity are the "plain" complexity C(x) and "prefix" complexity K(x); this gives two ways to define the set "R": RC and RK. (Of course, each different choice of universal Turing machine U in the definition of C and K yields another variant RCU or RKU.) Previous work on the power of "R" (for any of these variants [1,2,9]) has shown - BPP ⊆ {A : A≤ttpR}. - PSPACE ⊆ PR. - NEXP ⊆ NPR. Since these inclusions hold irrespective of low-level details of how "R" is defined, we have e.g.: NEXP ⊆ Δ10∩∩U NPRKU. (δ10 is the class of computable sets.) Our main contribution is to present the first upper bounds on the complexity of sets that are efficiently reducible to RKU. We show: - BPP ⊆ Δ10∩∩U{A : A≤ttpRKU} ⊆ PSPACE. - NEXP ⊆ Δ10 ∩∩U NPRKU ⊆ EXPSPACE. Hence, in particular, PSPACE is sandwiched between the class of sets Turingand truth-table-reducible to R. As a side-product, we obtain new insight into the limits of techniques for derandomization from uniform hardness assumptions.