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Some statement which implies the existence of Ramsey ultrafilters on \(\omega\) - MaRDI portal

Some statement which implies the existence of Ramsey ultrafilters on \(\omega\) (Q794638)

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scientific article; zbMATH DE number 3859109
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Some statement which implies the existence of Ramsey ultrafilters on \(\omega\)
scientific article; zbMATH DE number 3859109

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    Some statement which implies the existence of Ramsey ultrafilters on \(\omega\) (English)
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    1983
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    A filter \({\mathfrak F}\) on A is ample if for some infinite \(A_ 0\subset A\), \(A_ 0-x\) is finite for any x in \({\mathfrak F}\). \({\mathfrak F}\) is weakly ample if for every non principal ultrafilter \({\mathfrak U}\) on \(\omega\), \({\mathfrak F}\subset g({\mathfrak U})\) for some \(g:\omega \to A (g({\mathfrak U})=\{y\subset A| g^{-1}(y)\in {\mathfrak U}\}).\) Denote A\(n(\kappa)\) the statement: any non principal, weakly ample filter on \(\kappa\) is ample. It is trivial to check that: ample \(\Rightarrow\) weakly ample and \(An(\kappa)+\kappa \geq \lambda \Rightarrow An(\lambda).\) Puritz showed: 1) MA or C\(H\to An(c) (c=2^{\omega})\); 2) \(\neg An(2^ c)\); 3) A\(n(\omega)\) \(\to\) there are p-points on \(\omega\). The author shows: 1) \(An(\omega)\to An(c);\) 2) An(c) \(\to\) there are \(c^+\) Ramsey filters on \(\omega\) [the converse does not hold]; 3) \(cons(ZF)\to cons(ZFC+CH+2^{\omega_ 1}=\omega_ 3\pm An(\omega_ 2));\) 4) \(\kappa^{\omega}\geq 2^ c\to \neg An(\kappa).\)
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    Ramsey ultrafilters
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    ample filter
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    p-points
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    Ramsey filters
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