Delta-ring

In mathematics, a non-empty collection of sets

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 meant to highlight the ring's closure under countable intersection, in contrast to a 𝜎-ring which is closed under countable unions.

is a ring of sets but not a δ-ring.

δ-rings can be used instead of σ-algebras in the development of measure theory if one does not wish to allow sets of infinite measure.

Additionally, a semiring is a π-system where every complement

is equal to a finite disjoint union of sets in

A semialgebra is a semiring where every complement

is equal to a finite disjoint union of sets in

This mathematical analysis–related article is a stub.