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FALSE: every Borel subset of the real line is a countable union of countable intersections of open and closed sets

Statement

Every Borel subset of R belongs to the class obtained by taking countable intersections of open and closed sets and then countable unions of those intersections.

Facts & Assumptions

Given: The real line with its usual topology and the Borel classes of The countable Borel hierarchy and its limit convention. Assume the Axiom of Choice (The Axiom of Choice) for the local separation theorem below.

[L1]

In ZFC, every metrizable space containing a Cantor copy has a set in Π30∖Σ30 (Universal Borel sets and strictness on Cantor space).

[L3]

Cantor space C=2N is compact, and j(b)=∑k≥02b(k)3−k−1 continuously injects it into R (Cantor and Baire sequence spaces and coordinate codings, Continuous injections of sequence spaces into the real line).

Refutation

technique · contradiction
1.1givenconstruct

Every closed subset F of R is a countable intersection of open sets: if F≠∅, use Oj=⋃a∈F(a−1/(j+1),a+1/(j+1)). Points of F belong to every Oj; a point outside F has an interval disjoint from F, and hence misses Oj for sufficiently large j. For F=∅ use the constant empty sequence. Consequently any countable intersection of open and closed sets is a countable intersection of open sets: replace each closed factor by this explicit sequence, leave an open factor as its constant sequence, and enumerate pairs of indices by finite diagonals. Such a set is Π20, since its complement is a countable union of closed sets. Therefore every set in the claimed class is Σ30.

1.2L2L3L4construct

The map j in L3 constructs a Cantor copy in the real line. Put K=j[C] with its subspace topology. The real line is Hausdorff: two distinct reals s,t have disjoint open intervals of radius ∣s−t∣/3 around them. The subspace K is therefore also Hausdorff, by intersecting these separating intervals with K. As a map onto K, j is a continuous bijection: for open O in R, the preimage of K∩O is j−1[O]. L3 gives compactness of its domain, so L4 gives a homeomorphism C→K. L4 also makes K compact and closed in R. Thus this explicit ternary-series map supplies the required homeomorphic Cantor subset without choice.

2.1L1L2step 1.1step 1.2assume-contradischarge-contradiction∎

By L2 and step 1.2 the hypotheses of L1 hold for R, so choose D∈Π30(R)∖Σ30(R). This is a Borel set: the first three ranks are obtained from opens by complements and countable unions, operations preserving the Borel sigma-algebra. Suppose the statement were true. It would place D in the displayed class and thus, by step 1.1, in Σ30, contradicting its choice. Hence the statement is false. The only choice input is L1; the conversion in step 1.1 makes no simultaneous choices of representations.

Depends on

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Sources