Admissible ordinal
In set theory, an ordinal number α is an admissible ordinal if Lα is an admissible set ; in other words, α is admissible when α is a limit ordinal and Lα⊧Σ0-collection.
The first two admissible ordinals are ω and . Any regular uncountable cardinal is an admissible ordinal.
By a theorem of Sacks, the countable admissible ordinals are exactly those constructed in a manner similar to the Church-Kleene ordinal, but for Turing machines with oracles. One sometimes writes for the -th ordinal which is either admissible or a limit of admissibles; an ordinal which is both is called recursively inaccessible. There exists a theory of large ordinals in this manner that is highly parallel to that of large cardinals. But all these ordinals are still countable. Therefore, admissible ordinals seem to be the recursive analogue of regular cardinal numbers.
Notice that α is an admissible ordinal if and only if α is a limit ordinal and there does not exist a γ<α for which there is a Σ1 mapping from γ onto α. If M is a standard model of KP, then the set of ordinals in M is an admissible ordinal.
