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  1. Cofinal Types Below.Roy Shalev - forthcoming - Journal of Symbolic Logic:1-26.
    It is proved that for every positive integer n, the number of non-Tukey-equivalent directed sets of cardinality $\leq \aleph _n$ is at least $c_{n+2}$, the $(n+2)$ -Catalan number. Moreover, the class $\mathcal D_{\aleph _n}$ of directed sets of cardinality $\leq \aleph _n$ contains an isomorphic copy of the poset of Dyck $(n+2)$ -paths. Furthermore, we give a complete description whether two successive elements in the copy contain another directed set in between or not.
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  • Combinatorics of ultrafilters on Cohen and random algebras.Jörg Brendle & Francesco Parente - 2022 - Journal of Symbolic Logic 87 (1):109-126.
    We investigate the structure of ultrafilters on Boolean algebras in the framework of Tukey reducibility. In particular, this paper provides several techniques to construct ultrafilters which are not Tukey maximal. Furthermore, we connect this analysis with a cardinal invariant of Boolean algebras, the ultrafilter number, and prove consistency results concerning its possible values on Cohen and random algebras.
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  • Cofinal types on ω 2.Borisa Kuzeljevic & Stevo Todorcevic - 2023 - Mathematical Logic Quarterly 69 (1):92-103.
    In this paper we start the analysis of the class, the class of cofinal types of directed sets of cofinality at most ℵ2. We compare elements of using the notion of Tukey reducibility. We isolate some simple cofinal types in, and then proceed to find some of these types which have an immediate successor in the Tukey ordering of.
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  • Tukey order among ideals.Jialiang He, Michael Hrušák, Diego Rojas-Rebolledo & Sławomir Solecki - 2021 - Journal of Symbolic Logic 86 (2):855-870.
    We investigate the Tukey order in the class of Fσ ideals of subsets of ω. We show that no nontrivial Fσ ideal is Tukey below a Gδ ideal of compact sets. We introduce the notions of flat ideals and gradually flat ideals. We prove a dichotomy theorem for flat ideals isolating gradual flatness as the side of the dichotomy that is structurally good. We give diverse characterizations of gradual flatness among flat ideals using Tukey reductions and games. For example, we (...)
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