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2015 | OriginalPaper | Chapter

Computability on the Countable Ordinals and the Hausdorff-Kuratowski Theorem (Extended Abstract)

Author : Arno Pauly

Published in: Mathematical Foundations of Computer Science 2015

Publisher: Springer Berlin Heidelberg

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Abstract

While there is a well-established notion of what a computable ordinal is, the question which functions on the countable ordinals ought to be computable has received less attention so far. In order to remedy this, we explore various potential representations of the set of countable ordinals. An equivalence class of representations is then suggested as a standard, as it offers the desired closure properties. With a decent notion of computability on the space of countable ordinals in place, we can then state and prove a computable uniform version of the Hausdorff-Kuratowski theorem.

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Footnotes
1
Any algorithm attempting to compute \(\sup \) on \(\mathbf{COrd}_{\text {M}}\) needs to decide whether or not the result is 0 after finitely many steps – and this questions essentially is \(\text {LPO}\).
 
2
This result essentially is folklore.
 
3
In fact, it is sometimes claimed that it has to be done like that – the present work ought to disprove this.
 
4
This theorem is based on a personal communication by Vassilios Gregoriades.
 
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Metadata
Title
Computability on the Countable Ordinals and the Hausdorff-Kuratowski Theorem (Extended Abstract)
Author
Arno Pauly
Copyright Year
2015
Publisher
Springer Berlin Heidelberg
DOI
https://doi.org/10.1007/978-3-662-48057-1_32

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