Basic object in measure theory; set and a sigma-algebra
In mathematics, a measurable space or Borel space[1] is a basic object in measure theory. It consists of a set and a σ-algebra, which defines the subsets that will be measured.
Common measurable spaces
If is finite or countably infinite, the -algebra is most often the power set on so This leads to the measurable space
If is a topological space, the -algebra is most commonly the Borel -algebra so This leads to the measurable space that is common for all topological spaces such as the real numbers
Ambiguity with Borel spaces
The term Borel space is used for different types of measurable spaces. It can refer to
- any measurable space, so it is a synonym for a measurable space as defined above [1]
- a measurable space that is Borel isomorphic to a measurable subset of the real numbers (again with the Borel -algebra)[3]
Families of sets over
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Is necessarily true of or, is closed under:
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Directed by
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F.I.P.
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π-system
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Semiring
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Never
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Semialgebra (Semifield)
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Never
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Monotone class
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only if |
only if |
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𝜆-system (Dynkin System)
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only if
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only if or they are disjoint |
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Never
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Ring (Order theory)
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Ring (Measure theory)
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Never
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δ-Ring
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Never
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𝜎-Ring
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Never
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Algebra (Field)
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Never
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𝜎-Algebra (𝜎-Field)
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Never
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Dual ideal
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Filter
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Never |
Never |
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Prefilter (Filter base)
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Never |
Never |
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Filter subbase
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Never |
Never |
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Open Topology
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(even arbitrary ) |
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Never
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Closed Topology
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(even arbitrary ) |
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Never
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Is necessarily true of or, is closed under:
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directed downward
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finite intersections
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finite unions
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relative complements
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complements in
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countable intersections
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countable unions
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contains
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contains
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Finite Intersection Property
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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
are arbitrary elements of and it is assumed that
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