The class of Polish spaces has several universality properties, which show that there is no loss of generality in considering Polish spaces of certain restricted forms.
Every Polish space is homeomorphic to a Gδsubspace of the Hilbert cube, and every Gδ subspace of the Hilbert cube is Polish.
Every Polish space is obtained as a continuous image of Baire space; in fact every Polish space is the image of a continuous bijection defined on a closed subset of Baire space. Similarly, every compact Polish space is a continuous image of Cantor space.
Because of these universality properties, and because the Baire space has the convenient property that it is homeomorphic to, many results in descriptive set theory are proved in the context of Baire space alone.
Borel sets
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:
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 numbern, 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.
Each Borel set of a Polish space is classified in the Borel hierarchy based on how many times the operations of countable union and complementation must be used to obtain the set, beginning from open sets. The classification is in terms of countable ordinal numbers. For each nonzero countable ordinalα there are classes,, and.
A set is declared to be if and only if its complement is.
A set A is declared to be, δ > 1, if there is a sequence ⟨ Ai ⟩ of sets, each of which is for some λ < δ, such that.
A set is if and only if it is both and.
A theorem shows that any set that is or is, and any set is both and for all α > β. Thus the hierarchy has the following structure, where arrows indicate inclusion.
Regularity properties of Borel sets
Classical descriptive set theory includes the study of regularity properties of Borel sets. For example, all Borel sets of a Polish space have the property of Baire and the perfect set property. Modern descriptive set theory includes the study of the ways in which these results generalize, or fail to generalize, to other classes of subsets of Polish spaces.
Analytic and coanalytic sets
Just beyond the Borel sets in complexity are the analytic sets and coanalytic sets. A subset of a Polish space X is analytic if it is the continuous image of a Borel subset of some other Polish space. Although any continuous preimage of a Borel set is Borel, not all analytic sets are Borel sets. A set is coanalytic if its complement is analytic.
A set A is if there is a subset B of such that A is the projection of B to the first coordinate.
A set A is if there is a subset B of such that A is the projection of B to the first coordinate.
A set is if it is both and .
As with the Borel hierarchy, for each n, any set is both and The properties of the projective sets are not completely determined by ZFC. Under the assumption V = L, not all projective sets have the perfect set property or the property of Baire. However, under the assumption of projective determinacy, all projective sets have both the perfect set property and the property of Baire. This is related to the fact that ZFC proves Borel determinacy, but not projective determinacy. More generally, the entire collection of sets of elements of a Polish space X can be grouped into equivalence classes, known as Wadge degrees, that generalize the projective hierarchy. These degrees are ordered in the Wadge hierarchy. The axiom of determinacy implies that the Wadge hierarchy on any Polish space is well-founded and of length Θ, with structure extending the projective hierarchy.
A contemporary area of research in descriptive set theory studies Borel equivalence relations. A Borel equivalence relation on a Polish space X is a Borel subset of that is an equivalence relation on X.
