Hardness amplification within NP

  • Authors:
  • Ryan O'Donnell

  • Affiliations:
  • Institute for Advanced Study, Princeton, NJ

  • Venue:
  • Journal of Computer and System Sciences - Special issue on computational complexity 2002
  • Year:
  • 2004

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Abstract

In this paper we investigate the following question: if NP is slightly hard on average, is it very hard on average? We give a positive answer: if there is a function in NP which is infinitely often balanced and (1 - 1/poly(n))-hard for circuits of polynomial size, then there is a function in NP which is infinitely often (1/2 + n-1/2+ε)-hard for circuits of polynomial size. Our proof technique is to generalize the Yao XOR Lemma, allowing us to characterize nearly tightly the hardness of a composite function g(f(x1),....,f(xn)) in terms of: (i) the original hardness of f, and (ii) the expected bias of the function g when subjected to random restrictions. The computational result we prove essentially matches an information-theoretic bound.