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recursive ordinal : ウィキペディア英語版
recursive ordinal
In mathematics, specifically set theory, an ordinal \alpha is said to be recursive if there is a recursive well-ordering of a subset of the natural numbers having the order type \alpha.
It is trivial to check that \omega is recursive, the successor of a recursive ordinal is recursive, and the set of all recursive ordinals is closed downwards. The supremum of all recursive ordinals is called the Church-Kleene ordinal and denoted by \omega^_1. Indeed, an ordinal is recursive if and only if it is smaller than \omega^_1. Since there are only countably many recursive relations, there are also only countably many recursive ordinals. Thus, \omega^_1 is countable.
The recursive ordinals are exactly the ordinals that have an ordinal notation in Kleene's \mathcal.
==See also==

*Arithmetical hierarchy
*Large countable ordinals
*Ordinal notation

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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