English

Partitioning the real line into Borel sets

Logic 2024-05-22 v1

Abstract

For which infinite cardinals κ\kappa is there a partition of the real line R\mathbb R into precisely κ\kappa Borel sets? Hausdorff famously proved that there is a partition of R\mathbb R into 1\aleph_1 Borel sets. But other than this, we show that the spectrum of possible sizes of partitions of R\mathbb R into Borel sets can be fairly arbitrary. For example, given any AωA \subseteq \omega with 0,1A0,1 \in A, there is a forcing extension in which A={n:there is a partition of R into n Borel sets}A = \{ n :\, \text{there is a partition of }\mathbb R\text{ into }\aleph_n\text{ Borel sets}\}. We also look at the corresponding question for partitions of R\mathbb R into closed sets. We show that, like with partitions into Borel sets, the set of all uncountable κ\kappa such that there is a partition of R\mathbb R into precisely κ\kappa closed sets can be fairly arbitrary.

Cite

@article{arxiv.2112.00535,
  title  = {Partitioning the real line into Borel sets},
  author = {Will Brian},
  journal= {arXiv preprint arXiv:2112.00535},
  year   = {2024}
}
R2 v1 2026-06-24T07:59:43.097Z