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The Feferman - Levy model makes $\aleph_1$ singular by a cardinal collapse $\aleph_1 = \aleph_{\omega}^L$. Unless I've got something wrong, the same thing would work to make any well-orderable cardinal $\alpha$ cofinal in its well-ordered cardinal successor. Is that right?

The Feferman -Levy model also makes the continuum a countable union of countable sets. Does that generalize to Beth numbers, in the sense of successive power sets starting with $\omega$? For each finite $n$, are there models where $\beth_{n+1}$ is a union of $\beth_{n}$ many sets each smaller than or the same size as $\beth_{n}$?

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  • $\begingroup$ Are you familiar with the construction of the Feferman-Levy model (preferably with a slightly modern approach of symmetric extensions, rather than $L(G)$ for some generic set $G$)? Have you tried reiterating the construction with different variables? $\endgroup$
    – Asaf Karagila ♦
    Commented Oct 13, 2012 at 15:46
  • $\begingroup$ If you are not as familiar, mathoverflow.net/questions/100717/… has some information (including the comments). $\endgroup$
    – Asaf Karagila ♦
    Commented Oct 13, 2012 at 15:48
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    $\begingroup$ Hi Colin. I am not sure I understand the question in the first paragraph. Are you asking to preserve $\alpha$ and $\alpha^+$? ${\rm cof}(\alpha)$ and $\alpha^+$? Only $\alpha^+$? $\endgroup$
    – Andrés E. Caicedo
    Commented Oct 13, 2012 at 21:30
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    $\begingroup$ I figured. The issue in the first paragraph is that preserving a singular and collapsing its successor requires large cardinals, by Jensen's covering lemma. $\endgroup$
    – Andrés E. Caicedo
    Commented Oct 13, 2012 at 23:21
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    $\begingroup$ There is no actual use of constructibility. We could have equally have collapsed $\beth_n$'s to $\aleph_0$ and used the same process to ensure that $\beth_\omega$ is the new $\aleph_1$. $\endgroup$
    – Asaf Karagila ♦
    Commented Oct 15, 2012 at 17:43

1 Answer 1

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Posting this as an answer at Colin's request. The second paragraph of the question is addressed at this other MO question.

The answer to the question in the first paragraph is delicate, it depends on how much of the ground model we decide to preserve: $\alpha$ and $\alpha^+$? $\rm{cof}(\alpha)$ and $\alpha^+$? Only $\alpha^+$? The issue is that a straight generalization of Feferman-Lévy must fail just based on consistency strength considerations, because Jensen's covering lemma gives us that preserving a singular and collapsing its successor requires large cardinals (even in $\mathsf{ZF}$). For the covering lemma, see:

  • Devlin, Keith I.; Jensen, Ronald B. Marginalia to a theorem of Silver. In $\models$ISILC Logic Conference (Proc. Internat. Summer Inst. and Logic Colloq., Kiel, 1974), G. H. Müller, A. Oberschelp, and K. Potthoff, eds., pp. 115–142. Lecture Notes in Math., Vol. 499, Springer, Berlin, 1975. MR0480036 (58 #235)
  • Mitchell, William J. The covering lemma. In Handbook of set theory. Vols. 1, 2, 3, Kanamori, Foreman, eds., pp. 1497–1594, Springer, Dordrecht, 2010. MR2768697
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  • $\begingroup$ The reference in "MO question." is pointing to ams site. $\endgroup$
    – Eran
    Commented Oct 15, 2012 at 18:29
  • $\begingroup$ Typical. Thanks, it should be fixed now. $\endgroup$
    – Andrés E. Caicedo
    Commented Oct 15, 2012 at 19:11
  • $\begingroup$ Wait, how did my answer answer the second paragraph? It suggested that there is some model in which countable unions of countable sets have power sets which can be mapped onto sets of ordinals. This need not imply, methinks, that $\beth_{n+1}$ is a small union small sets. Gitik's model should be such example, though. Perhaps you wanted to link Francois' answer referred to by Colin in the comments? $\endgroup$
    – Asaf Karagila ♦
    Commented Oct 15, 2012 at 20:58

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