Concentrated sets
H. G. Eggleston · Mathematical Proceedings of the Cambridge Philosophical Society · 1967
A non-enumerable set of real numbers,Z, isconcentratedprovided that there exists an enumerable setKsuch that ifGis open andG⊂KthenZ−Gis at most enumerable.Zis said to be universally concentrated if we can take forKany enumerable setHfor which