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Criteria of constructibility for real numbers

Published online by Cambridge University Press:  12 March 2014

John Myhill*
Affiliation:
Yale University

Extract

The purpose of this paper is to prove two theorems and a conjecture (Conjecture II) announced in section 15 an earlier paper of the author's (cited as “CT”), and to compare them briefly with related results of Specker. Familiarity with both papers is assumed; the terminology of the former is used throughout. On two points however clarification of the usage of CT is in order, and to this chore we must first proceed.

A half-section is the lower half of a Dedekind cut; if the cut is rational, the half section is to include the rational corresponding to the real defined by the cut. A whole-section is the relation which holds between any member of the lower and any member of the upper half of some Dedekind cut. If the cut is rational the corresponding rational is to be a member of both halves.

A real number α is said to be approximate in K to any required number of decimal places if it is possible to define the predicatesx < α’, ‘xα’, ‘x > α’, and ‘xα’ (x rational) in K. In view of section 7 of CT this will mean that every true inequation between α and a terminating decimal will be provable in K.

Type
Research Article
Copyright
Copyright © Association for Symbolic Logic 1953

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References

1 A complete theory of natural, rational and real numbers, this Journal, vol. 15 (1950), pp. 185198Google Scholar. On page 196, line 19, after “definable” add “as a half-section”; line 21, after “definable” add “as a whole section”. The author has been unable to prove Conjecture I, and it is probably false.

2 Specker, Ernst, Nicht konstruktiv beweisbare Sätze der Analysis, this Journal, vol. 14 (1949), pp. 145158Google Scholar.

3 This theorem risks triviality because it is doubtful whether there exists such a bounded class.

4 The sign ‘<’ refers of course to the relation between rationals. We have not officially introduced this sign yet, but it is a routine chore to do so.

5 (Added August 4, 1952.) Since this paper was submitted, the author has seen an outline proof of what is essentially our Theorem IX in R. M. Robinson's review of Péter's, RószaRekursive Funktionen, this Journal, vol. 16 (1951), pp. 280282Google Scholar.