Skip to main content
1 of 5
Oscar Cunningham
  • 3.1k
  • 1
  • 26
  • 33

If there a universal way to force the Axiom of Choice to be true?

Given a model of set theory $V$ there are various ways to construct a model in which the Axiom of Choice holds, such as Gödel's constructible universe $L(V)$ or by using forcing. I'm wondering if any of these constructions have a nice universal property in the sense of category theory.

In particular are there any adjoints to the inclusion 2-functor $\mathcal{ZFC}\to\mathcal{ZF}$?

(Where $\mathcal{ZF}$ is the 2-category whose objects are either

  1. Models of ZF
  2. Toposes

and whose morphisms are either

  1. Geometric morphisms
  2. Logical functors
  3. Elementary embeddings

and where $\mathcal{ZFC}$ is the full subcategory on the objects that obey Choice.)

I think that the concept of the "minimal model" might allow one to construct a right adjoint which is similar in character to the constructable universe. Forcing goes "the other way" by adding sets rather than restricting them, so I suspect it might give a left adjoint.

Oscar Cunningham
  • 3.1k
  • 1
  • 26
  • 33