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Descriptive Set Theory is the study of definable subsets of Polish spaces, where definable is taken to mean from the Borel or projective hierarchies. Other topics include infinite games and determinacy, definable equivalence relations and Borel reductions between them, Polish groups, and effective descriptive set theory.

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Covering measure one sets by closed null sets

(The following question arose in a joint research with Adam Przeździecki and Boaz Tsaban.) For a $\sigma$-ideal $\mathcal{I}$ of subsets of the unit interval $[0,1]$, define $$\newcommand{\card}[1]{ …