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Extending Baire Property by countably many sets
Authors:Piotr Zakrzewski
Affiliation:Institute of Mathematics, University of Warsaw, ul. Banacha 2, 02-097 Warsaw, Poland
Abstract:We prove that if ZFC is consistent so is ZFC + ``for any sequence $(A_{n})$ of subsets of a Polish space $langle X,taurangle $ there exists a separable metrizable topology $tau'$ on $X$ with $mathbf{B}(X,tau)subseteqmathbf{B}(X,tau')$, $operatorname{MGR}(X,tau')capmathbf{B}(X,tau)=operatorname{MGR} (X,tau)capmathbf{B}(X,tau)$ and $A_{n}$ Borel in $tau'$ for all $n$.' This is a category analogue of a theorem of Carlson on the possibility of extending Lebesgue measure to any countable collection of sets. A uniform argument is presented, which gives a new proof of the latter as well.

Some consequences of these extension properties are also studied.

Keywords:Measure and category   Borel sets   Baire Property   $sigma$-algebra   $sigma$-ideal
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