Universal sets for ideals
Volume 66 / 2018
Abstract
We consider the notion of universal sets for ideals. We show that there exist universal sets of minimal Borel complexity for classical ideals like the null subsets of $2^\omega $ and the meager subsets of any Polish space, and demonstrate that the existence of such sets is helpful in establishing some facts about the real line in generic extensions. We also construct universal sets for $\mathcal {E}$, the $\sigma $-ideal generated by closed null subsets of $2^\omega $, and for some ideals connected with forcing notions: the $\mathcal K_\sigma $ subsets of $\omega ^{\omega }$ and the Laver ideal. We also consider Fubini products of ideals and show that there are $\Sigma ^0_3$ universal sets for $\mathcal N\otimes \mathcal M$ and $\mathcal M\otimes \mathcal N$.
