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Witnessing functions in bounded arithmetic and search problems

Published online by Cambridge University Press:  12 March 2014

Mario Chiari
Affiliation:
Mathematical Institute, Academy of Sciences, Žitná 25, Prague, 115 67, Czech Republic E-mail: chiari@prmat.math.unipr.it
Jan Krajíček
Affiliation:
Mathematical Institute and Institute of Computer Science, Academy of Sciences, Žitná 25, Prague, 115 67, Czech Republic E-mail: krajicek@math.cas.cz

Abstract

We investigate the possibility to characterize (multi)functions that are -definable with small i (i = 1, 2, 3) in fragments of bounded arithmetic T2 in terms of natural search problems defined over polynomial-time structures. We obtain the following results:

(1) A reformulation of known characterizations of (multi)functions that are and -definable in the theories and .

(2) New characterizations of (multi)functions that are and -definable in the theory .

(3) A new non-conservation result: the theory is not -conservative over the theory .

To prove that the theory is not -conservative over the theory , we present two examples of a -principle separating the two theories:

(a) the weak pigeonhole principle WPHP(a2, f, g) formalizing that no function f is a bijection between a2 and a with the inverse g,

(b) the iteration principle Iter(a, R, f) formalizing that no function f defined on a strict partial order ({0,…, a}, R) can have increasing iterates.

Information

Type
Research Article
Copyright
Copyright © Association for Symbolic Logic 1998

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