# Measurable space

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.

## Definition

Consider a set and a σ-algebra on . Then the tuple is called a measurable space.[2]

Note that in contrast to a measure space, no measure is needed for a measurable space.

## Example

Look at the set

One possible -Algebra would be

Then is a measurable space. Another possible -algebra would be the power set on :

With this, a second measurable space on the set is given by .

## Common measurable spaces

If is finite or countable infinite, the -algebra is most of the times 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]

## References

- Sazonov, V.V. (2001) [1994], "Measurable space", in Hazewinkel, Michiel (ed.),
*Encyclopedia of Mathematics*, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4 - Klenke, Achim (2008).
*Probability Theory*. Berlin: Springer. p. 18. doi:10.1007/978-1-84800-048-3. ISBN 978-1-84800-047-6. - Kallenberg, Olav (2017).
*Random Measures, Theory and Applications*. Probability Theory and Stochastic Modelling.**77**. Switzerland: Springer. p. 15. doi:10.1007/978-3-319-41598-7. ISBN 978-3-319-41596-3.