Stable ordinal

Stable ordinals are large ordinals defined using the notion of \(\Sigma_1\)-elementary substructure on levels of constructible universe. (invalid link)

For a natural number \(n\), let \(M \prec_{\Sigma_n} N\) denote the relation "\(M\) is a \(\Sigma_n\)-elementary substructure of \(N\).

There are special definitions for \(\Sigma_1\)-elementary substructures:
 * An ordinal \(\alpha\) is \((+\beta)\)-stable if \(L_\alpha\prec_{\Sigma_1}L_{\alpha+\beta}\).
 * An ordinal \(\alpha\) is \((^+)\)-stable if \(L_\alpha\prec_{\Sigma_1}L_\beta\) where \(\beta\) is next admissible ordinal after \(\alpha\).
 * An ordinal \(\alpha\) is \((^{++})\)-stable if \(L_\alpha\prec_{\Sigma_1}L_\beta\) where \(\beta\) is next admissible ordinal after the next admissible ordinal after \(\alpha\).
 * An ordinal \(\alpha\) is inaccessibly-stable if \(L_\alpha\prec_{\Sigma_1}L_\beta\) where \(\beta\) is next recursively inaccessible ordinal after \(\alpha\).
 * An ordinal \(\alpha\) is Mahlo-stable if \(L_\alpha\prec_{\Sigma_1}L_\beta\) where \(\beta\) is next recursively Mahlo ordinal after \(\alpha\).
 * An ordinal \(\alpha\) is doubly \((+\beta)\)-stable if there exists ordinal \(\gamma\) such that \(L_\alpha\prec_{\Sigma_1}L_\gamma\prec_{\Sigma_1}L_{\gamma+\beta}\).
 * An ordinal \(\alpha\) is nonprojectible if \(\sup\{\beta<\alpha|L_\beta\prec_{\Sigma_1}L_\alpha\}=\alpha\).
 * An ordinal \(\alpha\) is stable if \(L_\alpha\prec_{\Sigma_1}L\).

The general definition by Kripke is that for ordinals \(\alpha\) and \(\beta\) with \(\beta>\alpha\), \(\alpha\) is \(\beta\)-stable iff \(L_\alpha\prec_{\Sigma_1}L_\beta\).[​3]​ ​ The enumerating function of stable ordinals is continuous.

Suppose \(n\ge 1\). An ordinal \(\alpha\) is \(n\)-stable if \(L_\alpha\prec_{\Sigma_n}L\). For \(\rho>\alpha\), an ordinal \(\alpha\) is \(n\)-\(\rho\)-stable (also called "\((\rho,n)\)-stable" ) if \(L_\alpha\prec_{\Sigma_n}L_\rho\).

1-\(\rho\)-stability has the following properties: invalid link For an ordinal \(\alpha\), \(\alpha\) is nonprojectible iff \(L_\alpha\models\text{KP}\omega+\Sigma_1\text{-Sep}\), and "\(\alpha\) is nonprojectible" implies "\(\alpha\) is recursively Mahlo". For a limit ordinal \(\alpha\), \(L_\alpha\models\Sigma_n\text{-Sep}+\Sigma_n\text{-Coll}\) iff \(\forall x\in L_\alpha\exists M\in L_\alpha(x\subseteq M\land M\prec_{\Sigma_n}L_\alpha)\).
 * 1) If \(\alpha<\beta<\gamma\) and \(L_\alpha\prec_{\Sigma_1}L_\gamma\), then \(L_\alpha\prec_{\Sigma_1}L_\beta\).
 * 2) If \(L_\alpha\prec_{\Sigma_1}L_\beta\) and \(L_\beta\prec_{\Sigma_1}L_\gamma\), then \(L_\alpha\prec_{\Sigma_1}L_\gamma\).
 * 3) If \(\alpha<\beta\) and \(\beta\) is stable, then \(\alpha\) is stable iff \(L_\alpha\prec_{\Sigma_1}L_\beta\).
 * 4) If a set \(A\) is nonempty and \(\forall\alpha\in A(L_\alpha\prec_{\Sigma_1}L_\beta)\), then \(L_{\sup A}\prec_{\Sigma_1}L_\beta\).

Properties of stability:
 * (+1)-stable ordinals are exactly \(\Pi_0^1\)-reflecting ordinals.
 * \((^+)\)-stable ordinals are exactly \(\Pi_1^1\)-reflecting ordinals.
 * The least \(\Sigma_1^1\)-reflecting ordinal is greater than the least \(\Pi_1^1\)-reflecting ordinal, but every \((^{++})\)-stable ordinal is \(\Sigma_1^1\)-reflecting.
 * Stable ordinals are exactly \(\Sigma_2^1\)-reflecting ordinals, while the least of them is less than the least \(\Pi_2^1\)-reflecting ordinal.
 * \(\zeta\), the supremum of all eventually writable ordinals, is the least ordinal that is 2-\(\rho\)-stable for some \(\rho\); and \(\Sigma\), the supremum of all accidentally writable ordinals, is the least ordinal \(\rho\) such that \(\zeta\) is 2-\(\rho\)-stable.