Paper detail

Distinguishing perfect set properties in separable metrizable spaces

All spaces are assumed to be separable and metrizable. Our main result is that the statement "For every space $X$, every closed subset of $X$ has the perfect set property if and only if every analytic subset of $X$ has the perfect set property" is equivalent to $\mathfrak{b}>ω_1$ (hence, in particular, it is independent of $\mathsf{ZFC}$). This, together with a theorem of Solecki and an example of Miller, will allow us to determine the status of the statement "For every space $X$, if every $\mathbfΓ$ subset of $X$ has the perfect set property then every $\mathbfΓ'$ subset of $X$ has the perfect set property" as $\mathbfΓ,\mathbfΓ'$ range over all pointclasses of complexity at most analytic or coanalytic. Along the way, we define and investigate a property of independent interest. We will say that a subset $W$ of $2^ω$ has the Grinzing property if it is uncountable and for every uncountable $Y\subseteq W$ there exists an uncountable collection consisting of uncountable subsets of $Y$ with pairwise disjoint closures in $2^ω$. The following theorems hold. (1) There exists a subset of $2^ω$ with the Grinzing property. (2) Assume $\mathsf{MA}+\neg\mathsf{CH}$. Then $2^ω$ has the Grinzing property. (3) Assume $\mathsf{CH}$. Then $2^ω$ does not have the Grinzing property. The first result was obtained by Miller using a theorem of Todorčević, and is needed in the proof of our main result.