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The consistency of number theory via Herbrand's theorem1

Published online by Cambridge University Press:  12 March 2014

T. M. Scanlon
T. M. Scanlon*
Affiliation:
Princeton University, Princeton, New Jersey 08540
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Extract

That elementary number theory is consistent and can be given a metamathematical consistency proof has been well known since Gentzen's 1936 paper, and a number of different proofs of this result have since been offered. What is presented here is essentially a simplified and generalized version of the proof given by Ackermann in 1940 [1]; but the proof given here applies to systems formalized in standard quantification theory rather than in Hilbert's ε-calculus, and is based upon the analysis of quantificational reasoning given by Herbrand's Fundamental Theorem. Dreben and Denton sketch such a proof in [2], but at a crucial point they follow Ackermann in tying the strategy of their proof too closely to the standard model. This makes the proof more complex than it need be and restricts its application to systems with induction on the standard well-ordering. The present proof is both simpler and more general in that it applies to systems ZR of number theory with induction on arbitrary recursive well-orderings R. This generalization was first obtained by Tait in [11] using functionals of lowest type. Some technical devices employed below are similar to ones used by Tait, but while the proof-theoretic methods employed here are naturally characterizable in terms of functionals of lowest type the present proof avoids the introduction of such functionals into the languages studied.

Type
Research Article
Information
The Journal of Symbolic Logic , Volume 38 , Issue 1 , March 1973 , pp. 29 - 58
Copyright
Copyright © Association for Symbolic Logic 1973

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Footnotes

1

The work reported here forms part of a larger work to be co-authored with Burton Dreben and John Denton. It builds upon earlier unpublished work on consistency by Dreben and Denton, part of which is described in [2], but departs from their work in significant respects. One point of difference is discussed at the beginning of Appendix I. Another is the fact that the proof presented here applies to systems with induction on arbitrary recursive well-orderings. The possibility of obtaining this more general result by Herbrand-style methods was first established by W. D. Goldfarb through an entirely different strategy from the one employed here. I am indebted to numerous colleagues and students for helpful comments and suggestions, particularly to George Boolos, Denton, Dreben, Goldfarb, Harold Hodes, Judith Housman, Richard Shore, William Smith and Thomas Tymoczko. I am also indebted to the referee for numerous suggestions and corrections. Work on this paper was aided by NSF grant GS-2615.

References

REFERENCES

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