Delta-ring
Delta-ring
Ring closed under countable intersections
In mathematics, a non-empty collection of sets is called a Ξ΄-ring (pronounced "delta-ring") if it is closed under union, relative complementation, and countable intersection. The name "delta-ring" originates from the German word for intersection, "Durschnitt", which is meant to highlight the ring's closure under countable intersection, in contrast to a π-ring which is closed under countable unions.