Example of a topos that violates countable choice - MathOverflow

Example of a topos that violates countable choice

2011-11-02T06:18:20Z

In contrast, any topos that violates countable choice, of which there are plenty, must also violate internal COSHEP.

It doesn't give an example, and neither does the page on countable choice. So, what are these all-so-common examples?

Answer by Andreas Blass for Example of a topos that violates countable choice

2011-11-02T13:09:26Z

One sort of examples consists of the topoi of sets and functions obtained from models of ZF that violate countable choice. The original Cohen model is among these, and so are many others. Perhaps easier to understand are permutation models of ZFA (the variant of ZF that allows for atoms (= urelements)). The basic Fraenkel model, the second Fraenkel model, and Mostowski's linearly ordered model (probably the three best-known permutation models --- see Chapter 4 of Jech's book "The Axiom of Choice") all have infinite Dedekind-finite sets and therefore violate countable choice.

Answer by Mike Shulman for Example of a topos that violates countable choice

2011-11-04T23:41:08Z

If you're looking for a purely topos-theoretic model, I think you don't need to go through set theory (even though the end result may end up being basically equivalent). Look at the topos of continuous actions of the pro-completion of the integers, which is to say, the category of sets equipped with an automorphism all of whose orbits are finite. Here we have an N-indexed family of objects (one orbit of each cardinality) which are all inhabited, but whose product is empty -- hence the NNO is not internally projective.

Have you read P. Freyd's paper "The Axiom of Choice"?