delta ring

In mathematics, a non-empty collection of sets





R




{\displaystyle {\mathcal {R}}}
is called a δ-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 ment to highlight the ring's closure under countable intersection, in contrast to a 𝜎-ring which is closed under countable unions.

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