## PCF Hypotheses 2

November 28, 2011 at 12:17 | Posted in Uncategorized | Leave a commentAlmost all of the implications listed at the end of the last post follow immediately from the definitions involved. The one exception to this is the result that the Shelah Weak Hypothesis implies for any progressive set of regular cardinals . It isn’t a difficult result, but it uses “heavy machinery”: the Localization Theorem for pcf.

Suppose is a progressive set of regular cardinals satisfying . Let consist of the first elements of greater than (so is progressive).

If we define

then clearly is a singular cardinal of cofinality . Our goal is to obtain a violation of the Shelah Weak Hypothesis by proving

We mentioned the localization theorem earlier; we will be content just to quote it without proof:

Theorem 1 (Localization Theorem)Suppose is a progressive set of regular cardinals and does not have a weakly inaccessible point of accumulation. Then for any there is a set of cardinality at most such that .

The above is Theorem 3.4 on page 337 of The Book. Note that the theorem doesn’t require that is progressive; it demands only that is bounded in for every weakly inaccessible cardinal (of course, this is overkill for the application we have in mind).

Given , note

(This follows immediately upon consideration of the cofinality of where is any ultrafilter on disjoint to the ideal of bounded subsets of .)

By the Localization Theorem, it follows that for each there is a such that and

It follows easily that we can find an increasing and continuous sequence of cardinals such that

- , and
- for each .

For , let us define

and let be an ultrafilter on with

Now let be the least cardinal with

and an easy argument establishes that is a singular cardinal satisfying . Since , it follows that the sequence is strictly increasing, and therefore we have what we need.

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