Techniques of Admissible Recursion Theory

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1106

C.l Chong

Techniques of Admissible Recursion Theory

Springer-Verlag Berlin Heidelberg New York Tokyo 1984

Lecture Notes in Mathematics Edited by A. Oold and B. Eckmann Subseries: Harvard/MIT Adviser: G. Sacks

1106

C.l Chong

Techniques of Admissible Recursion Theory

Springer-Verlag Berlin Heidelberg New York Tokyo 1984

Author

Chi-Tat Chong Department of Mathematics, National University of Singapore Kent Ridge, Singapore 0511, Singapore

AMS Subject Classification (1980): 03060,03025 ISBN 3-540-13902-8 Springer-Verlag Berlin Heidelberg New York Tokyo ISBN 0-387-13902-8 Springer-Verlag New York Heidelberg Berlin Tokyo

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© by Springer-Verlag Berlin Heidelberg 1984 Printed in Germany Printing and binding: Beltz Offsetdruck, Hemsbach/Bergstr. 2146/3140-543210

FOREWORD

The generalization of recursion theory on the set w of natural numbers to infinite ordinals (now known as higher recursion theory) has a complicated history.

The guiding principle was that basic notions

such as computation, finiteness, recursion, recursiveness, relative recursiveness and effectiveness, which lie at the heart of the subject of classical recursion theory, should not be confined to the consideration of w (and hence countable structures) alone.

Indeed in various

parts of mathematical logic one had encountered notions and procedures which reminded one of effective computability in the generalized sense. however,

In order to develop a reasonable higher recursion theory, it was necessary to have a good intuition of the roles played

by objects such as finite sets and r.e. sets in the classical setting.

Thus for example in Kreisel [1971]

it is aptly pointed out

why in any reasonable generalization of recursion theory the notion of 'finiteness' ought to be central.

As early as 1938, Kleene had

introduced the idea of ordinal notations for natural numbers.

This

made it possible to discuss certain countably infinite ordinals (the recursive ordinals) in terms of the natural numbers they denote.

In

the 1950's and early 1960's various recursion­theoretic results on Church­Kleene wI (wICK), the least non­recursive ordinal, were derived, leading to the belief that a recursion theory on this ordinal (later christened 'metarecursion theory' by Kreisel and Sacks) could be developed modelled after classical recursion theory.

In particular

there should be a correct analog of relative recursiveness which would allow a positive solution to Post's problem for wICK subsequently confirmed in Sacks [1966a],

This was

[1966b].

It turns out that Godel's constructible universe L is an ideal

N structure in w