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  1. Cascades, order, and ultrafilters.Andrzej Starosolski - 2014 - Annals of Pure and Applied Logic 165 (10):1626-1638.
    We investigate mutual behavior of cascades, contours of which are contained in a fixed ultrafilter. This allows us to prove that the class of strict JωωJωω-ultrafilters, introduced by J.E. Baumgartner in [2], is empty. We translate the result to the language of <∞<∞-sequences under an ultrafilter, investigated by C. Laflamme in [17], and we show that if there is an arbitrary long finite <∞<∞-sequence under u , then u is at least a strict Jωω+1Jωω+1-ultrafilter.
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  • How high can Baumgartner’s $${\mathcal{I}}$$ I -ultrafilters lie in the P-hierarchy?Michał Machura & Andrzej Starosolski - 2015 - Archive for Mathematical Logic 54 (5-6):555-569.
    Under the continuum hypothesis we prove that for any tall P-ideal Ionω\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal{I} \,{\rm on}\,\, \omega}$$\end{document} and for any ordinal γ≤ω1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\gamma \leq \omega_1}$$\end{document} there is an I\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal{I}}$$\end{document}-ultrafilter in the sense of Baumgartner, which belongs to the class Pγ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal{P}_{\gamma}}$$\end{document} of the P-hierarchy of ultrafilters. Since the class (...)
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