4
$\begingroup$

Is there a model of $\mathsf{ZFC}$ such that for every cardinal $\beta > \aleph_0$ there is a cardinal $\alpha < \beta$ such that $|{\cal P}(\alpha)| >\beta$?

$\endgroup$
1
  • 4
    $\begingroup$ Take the limit of $\aleph_0,2^{\aleph_0},2^{2^{\aleph_0}},\dots$ $\endgroup$
    – Wojowu
    Nov 17, 2019 at 9:54

1 Answer 1

15
$\begingroup$

There cannot be such a model. Let for instance $\beta=\beth_\omega$, the supremum of $\beth_0=\aleph_0,\beth_1=2^{\beth_0},\beth_2=2^{\beth_1},\dots$, for any $\alpha<\beta$ we have $\alpha\leq\beth_n$ for some $n$, and hence $|\mathcal P(\alpha)|\leq\beth_{n+1}<\beta$.

The exact same argument shows that this proposition necessarily fails for all $\beta$ which are strong limit cardinals (i.e. $\kappa<\beta\implies 2^\kappa<\beta$). However, at least relative to large cardinals, it is consistent that those are the only points of failure - by a result of Woodin (apparently unpublished, discussed here) it is consistent with ZFC that for all $\alpha$ we have $2^{\aleph_\alpha}=\aleph_{\alpha+2}$. This implies that the strong limit cardinals are precisely $\aleph_\alpha$ with $\alpha$ limit. Hence if we take any infinite cardinal which is not a strong limit, it is of the form $\aleph_{\beta+1}$. Then we have $\aleph_\beta<\aleph_{\beta+1}$ but $2^{\aleph_\beta}>\aleph_{\beta+1}$.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.