In mathematics, the Church–Kleene ordinal, , named after Alonzo Church and S. C. Kleene, is a large countable ordinal. It is the set of all recursive ordinals and the smallest non-recursive ordinal. It is also the first ordinal which is not hyperarithmetical, and the first admissible ordinal after ω.
- 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
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