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Double-exponential inseparability of Robinson subsystem Q+

Published online by Cambridge University Press:  12 March 2014

Lavinia Egidi
Affiliation:
Dipartimento di Informatica, Università del Piemonte Orientale “A. Avogadro”, Viale T. Michel, 11, 15121 Alessandria, Italy, E-mail: lavinia.egidi@mfn.unipmn.it
Giovanni Faglia
Affiliation:
Dipartimento di Informatica, Università del Piemonte Orientale “A. Avogadro”, Viale T. Michel, 11, 15121 Alessandria, Italy, E-mail: giovanni.faglia@gmail.com

Abstract

In this work a double exponential time inseparability result is proven for a finitely axiomatizable first order theory Q+. The theory, subset of Presburger theory of addition S+, is the additive fragment of Robinson system Q. We prove that every set that separates Q+ from the logically false sentences of addition is not recognizable by any Turing machine working in double exponential time. The lower bound is given both in the non-deterministic and in the linear alternating time models.

The result implies also that any theory of addition that is consistent with Q+—in particular any theory contained in S+—is at least double exponential time difficult. Our inseparability result is an improvement on the known lower bounds for arithmetic theories.

Our proof uses a refinement and adaptation of the technique that Fischer and Rabin used to prove the difficulty of S+. Our version of the technique can be applied to any incomplete finitely axiomatizable system in which all of the necessary properties of addition are provable.

Type
Research Article
Copyright
Copyright © Association for Symbolic Logic 2011

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