### On the density of Hausdorff ultrafilters

by Bartoszynski and Shelah. [BrSh:826]

Logic Colloquium 2004, 2008

An ultrafilter U is Hausdorff if for any two functions f,g
in omega^omega, f(U)=g(U) iff f restriction X=g restriction
X for some X in U . We will show that the statement that
Hausdorff ultrafilters are dense in the Rudin-Keisler order is
independent of ZFC

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