Polish spaces up to Borel isomorphism
Two topological spaces^{} $X$ and $Y$ are Borel isomorphic (http://planetmath.org/BorelIsomorphism) if there is a Borel measurable function $f:X\to Y$ with Borel inverse^{}. Such a function is said to be a Borel isomorphism. The following result classifies all Polish spaces up to Borel isomorphism.
Theorem.
Every uncountable Polish space^{} is Borel isomorphic to $\mathrm{R}$ with the standard topology.
As the Borel $\sigma $algebra on any countable^{} metric space is just its power set^{}, this shows that every Polish space is Borel isomorphic to one and only one of the following.

1.
$\{1,2,\mathrm{\dots},n\}$ for some $n\ge 0$, with the discrete topology.

2.
$\mathbb{N}=\{1,2,\mathrm{\dots}\}$ with the discrete topology.

3.
$\mathbb{R}$ with the standard topology.
In particular, two Polish spaces are Borel isomorphic if and only if they have the same cardinality, and any uncountable Polish space has the cardinality of the continuum^{}.
Title  Polish spaces up to Borel isomorphism 

Canonical name  PolishSpacesUpToBorelIsomorphism 
Date of creation  20130322 18:47:00 
Last modified on  20130322 18:47:00 
Owner  gel (22282) 
Last modified by  gel (22282) 
Numerical id  6 
Author  gel (22282) 
Entry type  Theorem 
Classification  msc 54E50 
Related topic  CategoryOfPolishGroups 