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  1. Tight Eventually Different Families.Vera Fischer & Corey Bacal Switzer - 2024 - Journal of Symbolic Logic 89 (2):697-723.
    Generalizing the notion of a tight almost disjoint family, we introduce the notions of a tight eventually different family of functions in Baire space and a tight eventually different set of permutations of $\omega $. Such sets strengthen maximality, exist under $\mathsf {MA} (\sigma \mathrm {-centered})$ and come with a properness preservation theorem. The notion of tightness also generalizes earlier work on the forcing indestructibility of maximality of families of functions. As a result we compute the cardinals $\mathfrak {a}_e$ and (...)
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  • Isomorphism types of maximal cofinitary groups.Bart Kastermans - 2009 - Bulletin of Symbolic Logic 15 (3):300-319.
    A cofinitary group is a subgroup of Sym(ℕ) where all nonidentity elements have finitely many fixed points. A maximal cofinitary group is a cofinitary group, maximal with respect to inclusion. We show that a maximal cofinitary group cannot have infinitely many orbits. We also show, using Martin's Axiom, that no further restrictions on the number of orbits can be obtained. We show that Martin's Axiom implies there exist locally finite maximal cofinitary groups. Finally we show that there exists a uniformly (...)
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  • A Co-Analytic Cohen-Indestructible Maximal Cofinitary Group.Vera Fischer, David Schrittesser & Asger Törnquist - 2017 - Journal of Symbolic Logic 82 (2):629-647.
    Assuming that every set is constructible, we find a${\text{\Pi }}_1^1 $maximal cofinitary group of permutations of$\mathbb{N}$which is indestructible by Cohen forcing. Thus we show that the existence of such groups is consistent with arbitrarily large continuum. Our method also gives a new proof, inspired by the forcing method, of Kastermans’ result that there exists a${\text{\Pi }}_1^1 $maximal cofinitary group inL.
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