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

Abstract

It is consistent that the continuum be arbitrary large and no absolute $κ$-Borel set $X$ of density $κ$, $\aleph_1<κ<\mathfrak{c}$, condenses onto a compact metric space. It is consistent that the continuum be arbitrary large and any absolute $κ$-Borel set $X$ of density $κ$, $κ\leq\mathfrak{c}$, containing a closed subspace of the Baire space of weight $κ$, 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, $n\in A$.

Theorems & Definitions (12)

• Theorem 1.1
• Theorem 1.2
• Theorem 1.3
• Proposition 2.1
• proof
• Proposition 3.1
• proof
• Theorem 4.1
• proof
• Corollary 4.2