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.