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Asaf Karagila
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There's no known condition, and this isn't very well researched in the literature.

  1. Monro, in his paper on Dedekind finite sets, constructs a model with a proper class of Dedekind finite sets, so AC and DC cannot be forced by a set forcing, but in that model we can do a class forcing and restore choice.

  2. The Bristol model is locked between $L$ and $L[c]$, so it can be extended to a model of choice. It is not clear if this extension is a class forcing, and if so, is it definable in the model?

  3. If you take Monro's model, or even the Bristol model, and you "do a Feferman-Levy" symmetric extension to make $\omega_1$ singular, then you can still extend to get choice, but you must collapse $\omega_1$.

  4. In the Morris model, we cannot extend the model to a model of $\sf AC$ without adding ordinals because we have countable unions of countable sets that can be mapped on increasingly large sets. So, once we well-order everything, all cardinals must be below the continuum. Similarly in the Gitik model, everything has countable cofinality. This gives you an idea about necessary conditions. You need at least a proper class of regular cardinals. You need at least a bound on iterated power sets of sets that would have some fixed cardinality.

  5. As a side curiosity, Cohen, Solovay, and ultimately Friedman, showed that if $M$ is a countable model of $\sf ZFC$ of height $\alpha$, it can be extended to a model of $\sf ZF$ of the same height which is uncountable. These models cannot be extended back to models of $\sf ZFC$ without collapsing cardinals in the universe itself! Since a countable height implies countable when $\sf ZFC$ holds in the model.


Bibliography.

  1. G. P. Monro, Independence results concerning Dedekind-finite sets. J. Aust. Math. Soc., Ser. A 19, 35-46 (1975) (ZBL0298.02066).

  2. Asaf Karagila, The Bristol model: an abyss called a Cohen real. J. Math. Log. 18 (2), Article ID 1850008, 37 p. (2018) (ZBL1522.03215, arXiv:1704.06939).

  3. Asaf Karagila, The Morris model. Proc. Am. Math. Soc. 148 (3), 1311-1323 (2020) (ZBL1477.03212, arXiv:1811.10977).

Asaf Karagila
  • 39.9k
  • 8
  • 135
  • 283