On the first Banach problem, concerning condensations of absolute $\kappa$-Borel sets onto compacta

Abstract

It is consistent that the continuum be arbitrary large and no absolute $\kappa$-Borel set $X$ of density $\kappa$, ${\aleph }_1<\kappa <\mathfrak{c}$, condenses onto a compact metric space. It is consistent that the continuum be arbitrary large and any absolute $\kappa$-Borel set $X$ of density $\kappa$, $\kappa \le \mathfrak{c}$, containing a closed subspace of the Baire space of weight $\kappa$, condenses onto a compactum. In particular, applying Brian's results in model theory, we get the following unexpected result. Given any $A\subseteq \mathbb{N}$ with $1\in A$, there is a forcing extension in which every absolute ${\aleph }_n$-Borel set, containing a closed subspace of the Baire space of weight ${\aleph }_n$, condenses onto a compactum if, and only if, $n\in A$.

6 pages