proof of countable unions and intersections of analytic sets are analytic
Let (X,ℱ) be a paved space and (An)n∈ℕ be a sequence of ℱ-analytic sets
(http://planetmath.org/AnalyticSet2). We show that their union and intersection
is also analytic.
From the definition of ℱ-analytic sets, there exist compact paved spaces (Kn,𝒦n) and Sn∈(ℱ×𝒦n)σδ such that
An={x∈X:(x,y)∈Sn for some y∈Kn}. |
We start by showing that ⋂nAn is analytic. Let K=∏nKn and 𝒦=∏n𝒦n be the product paving, and πn:K→Kn be the projection map. Then x∈⋂nAn if and only if for each n there is a yn∈Kn with (x,yn)∈Sn. Equivalently, setting y=(y1,y2,⋯), then (x,y)∈⋂nπ-1n(Sn). However, this is in (ℱ×𝒦n)σδ and we can write,
⋂nAn=πX(⋂nπ-1n(Sn)), |
where πX:X×K→X is the projection map. As products of compact pavings are compact, (K,𝒦) is compact and it follows from the definition that ⋂nAn is ℱ-analytic.
We now show that ⋃nAn is analytic. Let K=∑nKn and 𝒦=∑n𝒦n be the direct sum paving, which is compact (http://planetmath.org/SumsOfCompactPavingsAreCompact). Also, write Sn=⋂∞m=1Tm,n for Tm,n∈(ℱ×𝒦n)σ.
We identify Kn with a subset of K, so that K is the union of the disjoint sets Kn.
Then x∈⋃nAn if and only if (x,y)∈Sn for some n and some y∈K,
⋃nAn=πX(⋃nSn). |
However, the fact that Kn1,Kn2 are disjoint for n1≠n2 says that Tm,n1,Tm,n2 are disjoint and, therefore,
⋃nSn=⋃n⋂mTm,n=⋂m⋃nTm,n∈(ℱ×𝒦)σδ. |
So ⋃nAn is ℱ-analytic.
Title | proof of countable unions and intersections of analytic sets are analytic |
---|---|
Canonical name | ProofOfCountableUnionsAndIntersectionsOfAnalyticSetsAreAnalytic |
Date of creation | 2013-03-22 18:46:19 |
Last modified on | 2013-03-22 18:46:19 |
Owner | gel (22282) |
Last modified by | gel (22282) |
Numerical id | 4 |
Author | gel (22282) |
Entry type | Proof |
Classification | msc 28A05 |