How statement and proof provenance work
The first chip identifies the source of the statement or construction; the second identifies the source of its local proof or verification.
- Literature-sourced: the exact statement appears in a cited source; only wording and notation differ.
- AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
- AI-generated: a genuinely novel statement formulated by AI, with no source for the claim itself.
These labels describe origin, not correctness: citations and verification chips remain separate evidence.
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 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.
In ZFC, every metrizable space containing a Cantor copy has a set in (Universal Borel sets and strictness on Cantor space).
The real line is metrizable (The absolute value makes a metric space: is a metric, its open balls are the intervals , and it is unbounded).
Cantor space is compact, and continuously injects it into (Cantor and Baire sequence spaces and coordinate codings, Continuous injections of sequence spaces into the real line).
Continuous images of compact spaces are compact, a continuous bijection from a compact space to a Hausdorff space is a homeomorphism, and compact subsets of Hausdorff spaces are closed (A continuous image of a compact space is compact; a continuous real-valued map on a nonempty compact space attains a maximum and a minimum; and a continuous bijection from a compact space to a Hausdorff space is a homeomorphism, In a Hausdorff space a point and a disjoint compact set, and two disjoint compact sets, have disjoint open neighbourhoods; hence every compact subset is closed, and in a compact Hausdorff space the compact subsets are exactly the closed ones).
Refutation
Every closed subset of is a countable intersection of open sets: if , use . Points of belong to every ; a point outside has an interval disjoint from , and hence misses for sufficiently large . For 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 , since its complement is a countable union of closed sets. Therefore every set in the claimed class is .
The map in L3 constructs a Cantor copy in the real line. Put with its subspace topology. The real line is Hausdorff: two distinct reals have disjoint open intervals of radius around them. The subspace is therefore also Hausdorff, by intersecting these separating intervals with . As a map onto , is a continuous bijection: for open in , the preimage of is . L3 gives compactness of its domain, so L4 gives a homeomorphism . L4 also makes compact and closed in . Thus this explicit ternary-series map supplies the required homeomorphic Cantor subset without choice.
By L2 and step 1.2 the hypotheses of L1 hold for , so choose . 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 in the displayed class and thus, by step 1.1, in , 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
- Universal Borel sets and strictness on Cantor space
- Continuous injections of sequence spaces into the real line
- Cantor and Baire sequence spaces and coordinate codings
- A continuous image of a compact space is compact; a continuous real-valued map on a nonempty compact space attains a maximum and a minimum; and a continuous bijection from a compact space to a Hausdorff space is a homeomorphism
- In a Hausdorff space a point and a disjoint compact set, and two disjoint compact sets, have disjoint open neighbourhoods; hence every compact subset is closed, and in a compact Hausdorff space the compact subsets are exactly the closed ones
- The absolute value makes $\mathbb{R}$ a metric space: $d(x,y) = |x-y|$ is a metric, its open balls are the intervals $(x-r, x+r)$, and it is unbounded
- The countable Borel hierarchy and its limit convention
- The Axiom of Choice
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
46 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- D. Marker, Descriptive Set Theory, Section 2, Corollary 2.38 (standard reference, not scraped)
- M. Christ, Math 202B Lecture 1, Comment on the Borel hierarchy (standard reference, not scraped)