Abstract

While it is known that intuitionistic ZF set theory formulated with Replacement, IZFR, does not prove Collection, it is a longstanding open problem whether IZFR and intuitionistic set theory ZF formulated with Collection, IZF, have the same proof-theoretic strength. It has been conjectured that IZF proves the consistency of IZFR. This paper addresses similar questions but in respect of constructive Zermelo–Fraenkel set theory, CZF. It is shown that in the latter context the proof-theoretic strength of Replacement is the same as that of Strong Collection and also that the functional version of the Regular Extension Axiom is as strong as its relational version.Moreover, it is proved that, contrary to IZF, the strength of CZF increases if one adds an axiom asserting that the trichotomous ordinals form a set.Unlike IZF, constructive Zermelo–Fraenkel set theory is amenable to ordinal analysis and the proofs in this paper make pivotal use thereof in the guise of well-ordering proofs for ordinal representation systems