The strength of replacement in weak arithmetic

  • Authors:
  • Stephen Cook;Neil Thapen

  • Affiliations:
  • Department of Computer Science, University of Toronto, Toronto, Ont., Canada;Department of Computer Science, University of Toronto, Toronto, Ont., Canada

  • Venue:
  • ACM Transactions on Computational Logic (TOCL)
  • Year:
  • 2006

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Abstract

The replacement (or collection or choice) axiom scheme BB(Γ) asserts bounded quantifier exchange as follows: ∀i a| ∃x aφ(i,x) → ∃w ∀i a|φ(i,[w]i), for φ in the class Γ of formulas. The theory S12 proves the scheme BB(Σb1), and thus in S12 every Σb1 formula is equivalent to a strict Σb1 formula (in which all non-sharply-bounded quantifiers are in front). Here we prove (sometimes subject to an assumption) that certain theories weaker than S12 do not prove either BB(Σb1) or BB(Σb0). We show (unconditionally) that V 0 does not prove BB(Σb0), where V0 (essentially IΣ1,b0) is the two-sorted theory associated with the complexity class AC0. We show that PV does not prove BB(Σb0), assuming that integer factoring is not possible in probabilistic polynomial time. Johannsen and Pollett introduced the theory C02 associated with the complexity class TC0, and later introduced an apparently weaker theory Δb1 − CR for the same class. We use our methods to show that Δb1 − CR is indeed weaker than C02, assuming that RSA is secure against probabilistic polynomial time attack.Our main tool is the KPT witnessing theorem.