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A proof-theoretic characterization of the primitive recursive set functions

  • Michael Rathjen (a1)


Let KP be the theory resulting from Kripke-Platek set theory by restricting Foundation to Set Foundation. Let G: VV (V ≔ universe of sets) be a Δ0-definable set function, i.e. there is a Δ0-formula φ(x, y) such that φ(x, G(x)) is true for all sets x, and V ⊨ ∀x∃!yφ(x, y). In this paper we shall verify (by elementary proof-theoretic methods) that the collection of set functions primitive recursive in G coincides with the collection of those functions which are Σ1-definable in KP + Σ1-Foundation + ∀x∃!yφ(x, y). Moreover, we show that this is still true if one adds Π1-Foundation or a weak version of Δ0-Dependent Choices to the latter theory.


Corresponding author

Department of Mathematics, Ohio State University, Columbus, Ohio 43210.


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