The question of the limits of recursive enumerability was first formulated by Sacks (1980) and investigated further in Sacks (198?). E-recursion or “set recursion”, as a natural generalization of Kleene recursion in normal objects of finite type, was introduced by Normann (1978) in order to facilitate the study of the degrees of functionals. We shall extend the work of Sacks on the question of how definable is the E-closure of an ordinal
α(written E( α)). We write gc( κ) to denote the largest τ < κsuch that L κ⊨ “τ is a cardinal” and cf (τ) for τ ∈ ON to denote the cofinality of τ.
In §1 we give the basic definitions and state the results of Silver and Friedman (1980) used by Sacks to show that if
E( α) = L κand is not Σ1-admissible and
P(gc( κ)) ∩ L κis indexical on L κand hence RE. We show in this case first that P(gc( κ)) ∩ L κindexical implies that L κis indexical (and hence RE).
In §2 we introduce the notion of a “nonstandard stage comparison” and use it to extend the definability result of §1 to show that this
L κis in fact REC. Finally we remark that E( α) is indexical if and only if E( α) is RE.