# The Church-Kleene ordinal $\omega_1^{ck}$

The *Church-Kleene* ordinal $\omega_1^{ck}$ is the supremum of the computable ordinals, where an ordinal $\alpha$ is *computable* if there is a computable relation $\triangle$ on $\mathbb{N}$ of order type $\alpha$, that is, such that $\langle\alpha,\lt\rangle\cong\langle\mathbb{N},\triangle\rangle$.

This ordinal is closed under all of the elementary ordinal arithmetic operations, such as successor, addition, multiplication and exponentiation.

The Church-Kleene ordinal is the least admissible ordinal.

## Relativized Church-Kleene ordinal

The Church-Kleene idea easily relativizes to oracles, where for any real $x$, we define $\omega_1^x$ to be the supremum of the $x$-computable ordinals. This is also the least admissible ordinal relative to $x$, and every countable admissible ordinal is $\omega_1^x$ for some $x$.

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