proof of equivalent definitions of analytic sets for Polish spaces
Let be a nonempty subset of a Polish space . Then, letting denote Baire space
and be any uncountable Polish space, we show that the following are equivalent
.
- 1.
is -analytic
(http://planetmath.org/AnalyticSet2).
- 2.
is the projection (http://planetmath.org/GeneralizedCartesianProduct) of a closed subset of onto .
- 3.
is the image (http://planetmath.org/DirectImage) of a continuous function
for some Polish space .
- 4.
is the image of a continuous function .
- 5.
is the image of a Borel measurable function .
- 6.
is the projection of a Borel subset of onto .
(1) implies (2):Let be the paving consisting of closed subsets of . The collection of -analytic sets
contains the Borel -algebra of (see countable unions and intersections of analytic sets are analytic) and, as the analytic sets are given by a closure operator
(http://planetmath.org/AnalyticSetsDefineAClosureOperator) it follows that it contains all analytic subsets of . So, any analytic subset of is -analytic.Then, there is a closed and a function such that
(see proof of equivalent definitions of analytic sets for paved spaces).For each let denote the closed subset of with . Then, we can rearrange the above expression to get where is the projection map and
It is easily seen that is a closed subset of for each , and therefore is closed, as required.
(2) implies (3):Suppose that for a closed subset of , where is the projection map. As the product of Polish spaces is Polish, and every closed subset of a Polish space is Polish, then will be a Polish space under the subspace topology. So, we can take and let be the restriction
of to .
(3) implies (4):Suppose that is the image of a continuous function , for a Polish space . As Baire space is universal for Polish spaces, there exists a continuous and onto (http://planetmath.org/Surjective) function . The result follows by taking .
(4) implies (5):Suppose that is the image of a continuous function . Since uncountable Polish spaces are all Borel isomorphic (see Polish spaces up to Borel isomorphism), there is a Borel isomorphism . The result follows by taking .
(5) implies (6):Suppose that is the image of a Borel measurable function , and let be its graph (http://planetmath.org/Graph2)
The projection of onto is equal to , so the result will follow once it is shown that is a Borel set.
Choose a countable and dense subset of , and let be a metric generating the topology
on . Then, for integers , denote the open ball
about of radius by . Since the form a dense set, for each . Let us define
which contains . Furthermore, since are Borel, are Borel sets.Suppose that . Then, for each , there is an such that and . So,
This holds for all , showing that and so . We have shown that is Borel, as required.
(6) implies (1):This is an immediate consequence of the result that projections of analytic sets are analytic.