Saturating Ultrafilters on N
Fremlin, D. H. ; Nyikos, P. J.
J. Symbolic Logic, Tome 54 (1989) no. 1, p. 708-718 / Harvested from Project Euclid
We discuss saturating ultrafilters on $\mathbf{N}$, relating them to other types of nonprincipal ultrafilter. (a) There is an $(\omega,\mathfrak{c})$-saturating ultrafilter on $\mathbf{N} \operatorname{iff} 2^\lambda \leq \mathfrak{c}$ for every $\lambda < \mathfrak{c}$ and there is no cover of $\mathbf{R}$ by fewer than $\mathfrak{c}$ nowhere dense sets. (b) Assume Martin's axiom. Then, for any cardinal $\kappa$, a nonprincipal ultrafilter on $\mathbf{N}$ is $(\omega,\kappa)$-saturating iff it is almost $\kappa$-good. In particular, (i) $p(\kappa)$-point ultrafilters are $(\omega,\kappa)$-saturating, and (ii) the set of $(\omega,\kappa)$-saturating ultrafilters is invariant under homeomorphisms of $\beta\mathbf{N\backslash N}$. (c) It is relatively consistent with ZFC to suppose that there is a Ramsey $p(\mathfrak{c})$-point ultrafilter which is not $(\omega,\mathfrak{c})$-saturating.
Publié le : 1989-09-14
Classification: 
@article{1183743010,
     author = {Fremlin, D. H. and Nyikos, P. J.},
     title = {Saturating Ultrafilters on N},
     journal = {J. Symbolic Logic},
     volume = {54},
     number = {1},
     year = {1989},
     pages = { 708-718},
     language = {en},
     url = {http://dml.mathdoc.fr/item/1183743010}
}
Fremlin, D. H.; Nyikos, P. J. Saturating Ultrafilters on N. J. Symbolic Logic, Tome 54 (1989) no. 1, pp.  708-718. http://gdmltest.u-ga.fr/item/1183743010/