Reflecting ordinal

Let \(A\) be a subclass of On, and \(m\) a natural number. A limit ordinal \(\alpha\) is said to be \(\Pi_m\)-reflecting on \(A\) if for any \(\Pi_n\) formula \(\phi(x)\) and any \(b\in L_\alpha\), if \((L_\alpha,\in)\models\phi(b)\) there exists a \(\beta \in A \cap \alpha\) such that \(b\in L_\beta\) and \((L_\beta,\in)\models\phi(b)\). A limit ordinal is said to be \(\Pi_m\)-reflecting ordinal if it is \(\Pi_m\)-reflecting on \(\textrm{On}\).

Although the short definition above looks like an ill-defined predicate requiring the "quantification of formulae", which is not allowed in the set theory itself because formulae are objects in the metatheory rather than the set theory, it is actually formalisable as a predicate in set theory itself using a non-trivial coding.

Examples

 * \(\Pi_3\)-reflecting ordinals are also known as recursively weakly compact ordinals.