Borel n-point sets in the plane for fixed n ≥ 2
Determine, for each fixed integer n ≥ 2, whether there exists a Borel subset M ⊆ R^2 that intersects every line l in exactly n points.
References
Nevertheless, the case of $\alpha_l$ being a fix natural number $n$ for any $n\geq2$ is still open.
— Partitions of $\mathbb{R}^3$ into unit circles with no well-ordering of the reals
(2501.03131 - Fatalini, 6 Jan 2025) in Subsubsection “Mazurkiewicz sets” (Subsection \ref{subsection: lit review paradoxical sets})