Constructive ordinal

In mathematics, specifically set theory, an ordinal ${\displaystyle \alpha }$ is said to be recursive if there is a recursive well-ordering of a subset of the natural numbers having the order type ${\displaystyle \alpha }$.

It is trivial to check that ${\displaystyle \omega }$ 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 ${\displaystyle \omega _{1}^{CK}}$. Indeed, an ordinal is recursive if and only if it is smaller than ${\displaystyle \omega _{1}^{CK}}$. Since there are only countably many recursive relations, there are also only countably many recursive ordinals. Thus, ${\displaystyle \omega _{1}^{CK}}$ is countable.

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