### The character spectrum of $\beta(N)$

by Shelah. [Sh:915]

Topology and its Applications, 2011

We show the consistency of: the set of regular cardinals
which are the character of some ultrafilter on omega can be quite
chaotic, in particular not only can be not convex but can
have many gaps.
We also deal with the set of pi-characters of
ultrafilters on omega

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