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14
votes
How to decompose an infinite set into two isomorphic ones without choice?
No it's not, you need to have at least some choice, for example that for every set $A$ we have $|A|+|A|=|A|$. For every cardinal $\kappa$, using ZFA with $\kappa^+$ atoms, you can create a permutation …
9
votes
1
answer
856
views
$\Pi_0^1$-weakly indescribable cardinals are exactly the regulars
Hi,
I'm not sure if I should ask here or over at math.stackexchange.com, but I think here it's a bit more fitting. This question stems from a homework problem:
Definition:
Given some class of formulas …