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An ordinal $\gamma$ is admissible if the $L_\gamma$ level of the constructible universe satisfies the Kripke-Platek axioms of set theory. The term was coined by Richard Platek in 1966.

The smallest admissible ordinal is often considered to be $\omega$, the least infinite ordinal. However, some authors include Infinity in the KP axioms, in which case $\omega_1^{CK}$,[1] the least non-computable ordinal, is the least admissible. More generally, for any real $x$, the least ordinal not computable from $x$ is denoted $\omega_1^x$, and is also admissible. Indeed, one has $L_{\omega_1^x}[x]\models\text{KP}$.

The smallest limit of admissible ordinals, $\omega_\omega^{CK}$, is not admissible.[1]

Equivalent definitions

The following properties are also equivalent to admissibility:

Computably inaccessible ordinal

An ordinal $\alpha$ is computably inaccessible, also known as recursively inaccessible, if it is admissible and a limit of admissible ordinals.[1] If \(f\) enumerates admissible ordinals, recursively inaccessible ordinals are exactly the ordinals \(\alpha\) where \(\alpha=f(\alpha)\).

Recursively Mahlo and further

An ordinal $α$ is recursively Mahlo iff for any $α$-recursive function $f : α → α$ there is an admissible $β < α$ closed under $f$.[1]

There are also recursively weakly compact i.e. $Π_3$-reflecting or 2-admissible ordinals.[1]

Higher admissibility

Some implications between admissibility-related conditions.

Admissibility has been extended using stronger collection axioms. One common formulation is that an ordinal $\alpha$ is $\Sigma_n$-admissible if $L_\alpha\vDash\textrm{RST}\cup\Sigma_n\textrm{-collection}$, where $\textrm{RST}$ denotes rudimentary set theory, i.e. Kripke-Platek set theory without the $\Sigma_0$-collection axiom.

\(\Sigma_n\)-admissible ordinals need not necessarily satisfy the \(\Sigma_n\)-separation schema. For example, the least \(\Sigma_2\)-admissible ordinal doesn't satisfy \(\Sigma_2\)-separation.

Here are some properties of $\Sigma_n$-admissibility:

  • $\Sigma_1$-admissibility is equivalent to $\Sigma_0$-admissibility.
  • For $n>1$, $\Sigma_n$-admissibility can be couched in terms of reflection onto sets of stable ordinals (Kranakis), and the smallest $\Sigma_n$-admissible ordinal is greater then the smallest nonprojectible ordinal and weaker variants of stable ordinals but smaller than the height of the minimal model of ZFC (if it exists).[1]

Cofinality and projectum

Two concepts used in the study of admissible ordinals are $\Sigma_1$-cofinality and $\Sigma_1$-projecta.

  • The $\Sigma_1$-cofinality of $\beta$ is the least $\xi$ such that there exists a $\Sigma_n$-definable function mapping $\xi$ cofinally into $\beta$. [1]
  • The $\Sigma_n$-projectum of $\beta$ is equal to the least $\delta$ such that some $\Sigma_n(L_\alpha)$-definable function maps a subset of $\delta$ onto $L_\beta$ [2]. (However note that when using the Jensen hierarchy instead of the hierarchy $L$, as the original source does, that behavior may change)
    • Alternatively, when $n=1$, the $\Sigma_1$-projectum of $\beta$ has been given as the least $\gamma\le\beta$ such that a $\beta$-recursive one-to-one function $f:\beta\rightarrow\gamma$ exists. [3][Barwise, p.157]

$\Delta_n$-projecta are similar to $\Sigma_n$-projecta, except that its behavior lacks the involvement of a bounded subset of $\delta$, employing just the ordinal $\delta$ instead. (Compare Σn: [4], Δn: [5])


  • $\beta$ is admissible iff $\Sigma_1\textrm{-cof}(\beta)=\beta$ [6].
    • Note that although admissibility is considered to be "recursive regularity"[ citation needed ], $\Sigma_1\textrm{-cof}$ behaves differently with respect to admissibles than $\textrm{cof}$ does with respect to regular cardinals. For example, $\textrm{cof}(\omega_1\times 2)=\omega_1$, however $\Sigma_1\textrm{-cof}(\omega_1^{CK}\times 2)=\omega$. (This is because there's a one-to-one map $f:\omega_1^{CK}\rightarrow\omega$ that's $\omega_1^{CK}$-recursive, and $f$ is also $\omega_1^{CK}\times 2$-recursive)
  • $\beta$ is nonprojectible iff $\Sigma_1\textrm{-proj}(\beta)=\beta$ [7].
    • For the alternative definition of the $\Sigma_1$-projectum, compare to Rathjen's description of nonprojectible ordinals [8].
  • A more fine but extendable result, if we assume $n>1$ and $\omega\beta=\beta$, then $\Sigma_1\textrm{-proj}(\beta)>\omega$ iff $\beta$ begins a $\Sigma_n$-gap. [9]. Similarly, if $\Delta_n\textrm{-proj}(\beta)>\omega$ then $\beta$ begins a $\Delta_n$-gap.
  • When $\beta$ is admissible[ citation needed ], $L_\beta\vDash``\Sigma_1\textrm{-cof}(\beta)\textrm{ is a cardinal}"$ and $L_\beta\vDash``\Sigma_1\textrm{-proj}(\beta)\textrm{ is a cardinal}"$. [10]
  • Applying a result from here, $L_{\Sigma_1\textrm{-cof}(\beta)}$$\prec_{\Sigma_1}$$L_\beta$ and $L_{\Sigma_1\textrm{-proj}(\beta)}\prec_{\Sigma_1}L_\beta$.


  1. Madore, David. A zoo of ordinals. , 2017. www   bibtex
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Sometimes, some unintuitive patterns arise in projecta of an ordinal, such as $\Sigma_1\textrm{-proj}(\alpha)=\Sigma_2\textrm{-proj}(\alpha)>\Sigma_3\textrm{-proj}(\alpha)$. In fact, for any binary string, there exists some ordinal $\alpha$ whose sequence $\{\Sigma_k\textrm{-proj}\alpha):0\le k\le n\}$ has pairwise comparisons $>$, $=$ each determined by that string. citation needed