Monotone versus positive

  • Authors:
  • Miklos Ajtai;Yuri Gurevich

  • Affiliations:
  • IBM Research, San Jose, CA;Univ. of Michigan, Ann Arbor

  • Venue:
  • Journal of the ACM (JACM)
  • Year:
  • 1987

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Abstract

In connection with the least fixed point operator the following question was raised: Suppose that a first-order formula P(P) is (semantically) monotone in a predicate symbol P on finite structures. Is P(P) necessarily equivalent on finite structures to a first-order formula with only positive occurrences of P? In this paper, this question is answered negatively. Moreover, the counterexample naturally gives a uniform sequence of constant-depth, polynomial-size, monotone Boolean circuits that is not equivalent to any (however nonuniform) sequence of constant-depth, polynomial-size, positive Boolean circuits.