The class of Borel sets of a topological space X consists of all sets in the smallest σ-algebra containing the open sets of X. This means that the Borel sets of X are the smallest collection of sets such that:
- Every open subset of X is a Borel set.
- If A is a Borel set, so is . That is, the class of Borel sets are closed under complementation.
- If An is a Borel set for each natural number n, then the union is a Borel set. That is, the Borel sets are closed under countable unions.
A fundamental result shows that any two uncountable Polish spaces X and Y are Borel isomorphic: there is a bijection from X to Y such that the preimage of any Borel set is Borel, and the image of any Borel set is Borel. This gives additional justification to the practice of restricting attention to Baire space and Cantor space, since these and any other Polish spaces are all isomorphic at the level of Borel sets.
Read more about this topic: Descriptive Set Theory
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