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  1.  9
    Countably perfectly Meager sets.Roman Pol & Piotr Zakrzewski - 2021 - Journal of Symbolic Logic 86 (3):1214-1227.
    We study a strengthening of the notion of a perfectly meager set. We say that a subset A of a perfect Polish space X is countably perfectly meager in X, if for every sequence of perfect subsets $\{P_n: n \in \mathbb N\}$ of X, there exists an $F_\sigma $ -set F in X such that $A \subseteq F$ and $F\cap P_n$ is meager in $P_n$ for each n. We give various characterizations and examples of countably perfectly meager sets. We prove (...)
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  2.  16
    On countably perfectly meager and countably perfectly null sets.Tomasz Weiss & Piotr Zakrzewski - 2024 - Annals of Pure and Applied Logic 175 (1):103357.
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    On universal semiregular invariant measures.Piotr Zakrzewski - 1988 - Journal of Symbolic Logic 53 (4):1170-1176.
    We consider countably additive, nonnegative, extended real-valued measures which vanish on singletons. Such a measure is universal on a set X iff it is defined on all subsets of X and is semiregular iff every set of positive measure contains a subset of positive finite measure. We study the problem of existence of a universal semiregular measure on X which is invariant under a given group of bijections of X. Moreover we discuss some properties of universal, semiregular, invariant measures on (...)
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