What is the theory ZFC without power set?

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V. Gitman, J. D. Hamkins, and T. A. Johnstone, “What is the theory $\mathsf {ZFC}$ without power set?,” MLQ Math. Log. Q., vol. 62, no. 4-5, pp. 391–406, 2016.

Set theory without the power set axiom is used in arguments and constructions throughout the subject and is usually described simply as having all the axioms of $\rm{ZFC}$ except for the power set axiom. This theory arises frequently in the large cardinal theory of iterated ultrapowers, for example, and perhaps part of its attraction is an abundance of convenient natural models, including $\langle H_\kappa,{\in}\rangle$ for any uncountable regular cardinal $\kappa$, where $H_\kappa$ consists of sets with hereditary size less than $\kappa$. When prompted, many set theorists offer a precise list of axioms: extensionality, foundation, pairing, union, infinity, separation, replacement and choice. Let us denote by $\rm{ZFC}{-}$ the theory having the axioms listed above with the axiom of choice taken to mean Zermelo's well-ordering principle, which then implies Zorn's Lemma as well as the existence of choice-functions. These alternative formulations of choice are not all equivalent without the power set axiom as is proved by Zarach in [1], in particular there are models of $\rm{ZF}^-$ in which choice-functions exist but Zermelo's well-ordering principle fails. Zarach initiated the program of establishing unintuitive consequences of set theory without power set, which we carry on in this article.

In this article, we shall prove that this formulation of set theory without the power set axiom is weaker than may be supposed and is inadequate to prove a number of basic facts that are often desired and applied in its context. Specifically, we shall prove that the following behavior can occur with $\rm{ZFC}{-}$ models.

Nevertheless, these deficits of $\rm{ZFC}{-}$ are completely repaired by strengthening it to the theory $\rm{ZFC}^-$, obtained by using collection rather than replacement in the axiomatization above.


  1. A. Zarach, “Unions of ${\rm ZF}^{-}$-models which are themselves ${\rm ZF}^{-}$-models,” in Logic Colloquium ’80 (Prague, 1980), vol. 108, Amsterdam: North-Holland, 1982, pp. 315–342.
  2. A. M. Zarach, “Replacement $\nrightarrow$ collection,” in Gödel ’96 (Brno, 1996), vol. 6, Berlin: Springer, 1996, pp. 307–322.
  3. H. Gaifman, “Elementary embeddings of models of set-theory and certain subtheories,” in Axiomatic set theory (Proc. Sympos. Pure Math., Vol. XIII, Part II, Univ. California, Los Angeles, Calif., 1967), Providence R.I.: Amer. Math. Soc., 1974, pp. 33–101.