# Standard Borel space

In mathematics, a standard Borel space is the Borel space associated to a Polish space. Discounting Borel spaces of discrete Polish spaces, there is, up to isomorphism of measurable spaces, only one standard Borel space.

## Formal definition

A measurable space ${\displaystyle (X,\Sigma )}$ is said to be "standard Borel" if there exists a metric on ${\displaystyle X}$ that makes it a complete separable metric space in such a way that ${\displaystyle \Sigma }$ is then the Borel σ-algebra.[1] Standard Borel spaces have several useful properties that do not hold for general measurable spaces.

## Properties

• If ${\displaystyle (X,\Sigma )}$ and ${\displaystyle (Y,T)}$ are standard Borel then any bijective measurable mapping ${\displaystyle f:(X,\Sigma )\to (Y,\mathrm {T} )}$ is an isomorphism (that is, the inverse mapping is also measurable). This follows from Souslin's theorem, as a set that is both analytic and coanalytic is necessarily Borel.
• If ${\displaystyle (X,\Sigma )}$ and ${\displaystyle (Y,T)}$ are standard Borel spaces and ${\displaystyle f:X\to Y}$ then ${\displaystyle f}$ is measurable if and only if the graph of ${\displaystyle f}$ is Borel.
• The product and direct union of a countable family of standard Borel spaces are standard.
• Every complete probability measure on a standard Borel space turns it into a standard probability space.

## Kuratowski's theorem

Theorem. Let ${\displaystyle X}$ be a Polish space, that is, a topological space such that there is a metric ${\displaystyle d}$ on ${\displaystyle X}$ that defines the topology of ${\displaystyle X}$ and that makes ${\displaystyle X}$ a complete separable metric space. Then ${\displaystyle X}$ as a Borel space is Borel isomorphic to one of (1) ${\displaystyle \mathbb {R} ,}$ (2) ${\displaystyle \mathbb {Z} }$ or (3) a finite space. (This result is reminiscent of Maharam's theorem.)

It follows that a standard Borel space is characterized up to isomorphism by its cardinality,[2] and that any uncountable standard Borel space has the cardinality of the continuum.

Borel isomorphisms on standard Borel spaces are analogous to homeomorphisms on topological spaces: both are bijective and closed under composition, and a homeomorphism and its inverse are both continuous, instead of both being only Borel measurable.