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Borel sets without perfectly many overlapping translations

Rosłanowski Andrzej, Shelah Saharon

Reports on Mathematical Logic

2019

Vol. 54

3--43

We study the existence of Borel sets B ⊆ ω2 admitting a sequence ηα : α<λ of distinct elements of ω2 such that (ηα +B)∩(ηβ +B) ≥ 6 for all α, β < λ but with no perfect set of such η’s. Our result implies that under the Martin Axiom, if ℵα < c, α<ω1 and 3 ≤ ι<ω, then there exists a Σ0 2 set B ⊆ ω2 which has ℵα many pairwise 2ι–nondisjoint translations but not a perfect set of such translations. Our arguments closely follow Shelah [7, Section 1].

On the Class of Perfectly Null Sets and Its Transitive Version

Korch M., Weiss T.

Bulletin of the Polish Academy of Sciences. Mathematics

2016

Vol. 64, no. 1

1--20

We introduce two new classes of special subsets of the real line: the class of perfectly null sets and the class of sets which are perfectly null in the transitive sense. These classes may play the role of duals to the corresponding classes on the category side. We investigate their properties and, in particular, we prove that every strongly null set is perfectly null in the transitive sense, and that it is consistent with ZFC that there exists a universally null set which is not perfectly null in the transitive sense. Finally, we state some open questions concerning the above classes. Although the main problem of whether the classes of perfectly null sets and universally null sets are consistently different remains open, we prove some results related to this question.