Church-Kleene Ordinal
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Church%E2%80%93Kleene Ordinal

In mathematics, the Church-Kleene ordinal, ${\displaystyle \omega _{1}^{\mathrm {CK} }}$, named after Alonzo Church and S. C. Kleene, is a large countable ordinal. It is the set of all recursive ordinals and consequently the smallest non-recursive ordinal. Since the successor of a recursive ordinal is recursive, the Church-Kleene ordinal is a limit ordinal. It is also the first ordinal which is not hyperarithmetical, and the first admissible ordinal after ?.

## References

• Church, Alonzo; Kleene, S. C. (1937), "Formal definitions in the theory of ordinal numbers.", Fundamenta mathematicae, Warszawa, 28: 11-21, JFM 63.0029.02
• Church, Alonzo (1938), "The constructive second number class", Bull. Amer. Math. Soc., 44 (4): 224-232, doi:10.1090/S0002-9904-1938-06720-1
• Kleene, S. C. (1938), "On Notation for Ordinal Numbers", The Journal of Symbolic Logic, The Journal of Symbolic Logic, Vol. 3, No. 4, 3 (4): 150-155, doi:10.2307/2267778, JSTOR 2267778
• Rogers, Hartley (1987) [1967], The Theory of Recursive Functions and Effective Computability, First MIT press paperback edition, ISBN 978-0-262-68052-3