How many clouds cover the plane? James H. Schmerl (2003) Fundamenta Mathematicae The plane can be covered by n + 2 clouds iff 2 ℵ ₀ ≤ ℵ ₙ .
How round is a circle? Constructions of double and circular planar near-rings. Veldsman, Stefan (2003) Beiträge zur Algebra und Geometrie