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One of the famous applications of pcf theory is that if $\aleph_\omega$ is a strong limit cardinal then $2^{\aleph_\omega}<\aleph_{\omega_4}$. I'm curious whether any weaker result with the same flavor can be gotten without pcf theory. It's a bit tricky to phrase this precisely, but here's one attempt: can the following result be proved in $\mathsf{ZFC}$ without pcf theory?

$(*)\quad$ Suppose $\aleph_\omega$ is a strong limit cardinal. Then there is some cardinal $\kappa$ such that for every cardinal-preserving set-generic extension $V[G]$ satisfying "$\aleph_\omega$ is a strong limit cardinal" we have $2^{\aleph_\omega}<\kappa$.

Certainly $(*)$ is implied by Shelah's result, since if $V$ and $V[G]$ have the same cardinals they agree on $\aleph_{\omega_4}$. But in principle it seems like it could be much weaker.

I'd also be interested in pcf-free proofs of further weakenings of $(*)$; to me, the most natural weakening seems to be gotten by restricting attention to some particularly nice class of forcings. (Basically, that "tame" forcings can't move $2^{\aleph_\omega}$ too much while keeping it a strong limit.)

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    $\begingroup$ Shelah's original proof that the given power is less than $\aleph_{(2^{\aleph_0})^+}$ in his "Proper forcing" book is such an example, I think. $\endgroup$ Commented Jan 22, 2021 at 9:40
  • $\begingroup$ Somehow related question is part (1) of the following: mathoverflow.net/questions/137318/… $\endgroup$ Commented Jan 22, 2021 at 9:43
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    $\begingroup$ Here's a PCF free argument: suppose that $2^{\aleph_\omega}>\aleph_{\omega_4}$ while still being a strong limit cardinal. Then by a result of Shelah ZFC is inconsistent. $\endgroup$ Commented Jan 22, 2021 at 10:04
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    $\begingroup$ The Shelah $2^{\aleph_\omega}<\aleph_{(2^{\aleph_0})^+}$ argument is quite complicated and if you look at it, it contains many elements of the (later) pcf theory. On the other hand the Galvin-Hajnal $2^{\aleph_{\omega_1}}<\aleph_{(2^{\aleph_1})^+}$ has a reasonable proof. $\endgroup$ Commented Jan 22, 2021 at 14:05
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    $\begingroup$ @David: smbc-comics.com/comic/2008-02-26 $\endgroup$ Commented Jan 22, 2021 at 15:09

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