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Assuming countable choice, every Borel subset of is Lebesgue measurable
Statement
Let and assume the Axiom of Countable Choice (The Axiom of Countable Choice ()). Then
every Borel subset of (The Borel sigma-algebra of a topological space) is Lebesgue measurable (Lebesgue measurable sets, the family , and the restricted set function ). In particular every open set, every closed set and every countable intersection of open sets is Lebesgue measurable.
Facts & Assumptions
Given: A natural number and the Axiom of Countable Choice.
Assuming countable choice, is a sigma-algebra on and every elementary set is Lebesgue measurable (Assuming countable choice, is a sigma-algebra containing every elementary set and is a complete measure extending elementary volume).
, where is the family of half-open boxes and the family of elementary sets (The sigma-algebra generated by the half-open boxes of is the Borel sigma-algebra).
At every half-open box is elementary (Elementary sets: the finite unions of half-open boxes in ).
is the unique smallest sigma-algebra on containing (Nonempty intersections of sigma-algebras are sigma-algebras, so the generated sigma-algebra exists and is minimal, The sigma-algebra generated by a family of sets).
The Borel sigma-algebra of is the sigma-algebra generated by its open sets (The Borel sigma-algebra of a topological space); a sigma-algebra on is an algebra of subsets closed under countable unions (Sigma-algebras).
The Axiom of Countable Choice says that for every family of nonempty sets indexed by there is a function with domain such that for every (The Axiom of Countable Choice ()).
Proof
Under countable choice is a sigma-algebra on containing every elementary set, hence containing the family of half-open boxes.
Since is the smallest sigma-algebra containing , step 1.1 gives , and ; open sets, closed sets and countable intersections of open sets are Borel.
Depends on
- Assuming countable choice, $\mathcal{L}(\mathbb{R}^n)$ is a sigma-algebra containing every elementary set and $\lambda_n$ is a complete measure extending elementary volume
- The sigma-algebra generated by the half-open boxes of $\mathbb{R}^n$ is the Borel sigma-algebra
- The Borel sigma-algebra of a topological space
- Sigma-algebras
- The sigma-algebra generated by a family of sets
- Nonempty intersections of sigma-algebras are sigma-algebras, so the generated sigma-algebra exists and is minimal
- Lebesgue measurable sets, the family $\mathcal{L}(\mathbb{R}^n)$, and the restricted set function $\lambda_n$
- Elementary sets: the finite unions of half-open boxes in $\mathbb{R}^n$
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
Used by
- Every subset of ℝⁿ has a G_δ measurable hull of the same outer measure Corollary
- L(ℝⁿ) is exactly the completion of the restriction of λₙ to the Borel sets Corollary
- There is a Lebesgue measurable subset of ℝ that is not Borel Corollary
- A nonintegrable observable with divergent ergodic averages Counterexample
- A null set can fail to be the discontinuity set of any function Counterexample
- Neumann Poisson data require a flux compatibility equation Counterexample
- Not every compact set is conformally removable Counterexample
- Point evaluation is unbounded below the Sobolev continuity threshold Counterexample
- Pointwise modification can destroy path continuity Counterexample
- The critical Riesz potential can diverge and be essentially unbounded Counterexample
- The indicator of a fat Cantor set is upper semicontinuous and equal almost everywhere to no Riemann integrable function Counterexample
- Lebesgue inner measure on the real line Definition
- The Bergman space A²(Ω) and the Bergman kernel Definition
- The one-dimensional torus and its normalized Haar integral Definition
- The polar surface set function on the unit sphere Definition
- A clipped affine function keeps its zero region Example
- An open dense set of measure less than 1 is the monotone L¹-limit of Riemann integrable indicators, but its indicator is not Riemann integrable Example
- Matching C¹ pieces across a hyperplane have no jump derivative Example
- Newtonian potential of radial compact data Example
- Sharp Sobolev threshold for a radial power Example
- The absolute value has a weak first derivative Example
- The positive-type Gaussian on the real line and its cyclic model Example
- Birkhoff's theorem requires integrability False statement
- A measurable set of positive finite measure occupies more than any prescribed proportion of some dyadic cube Lemma
- A separated characteristic disk has a minimal nonidentity simple cycle Lemma
- A shear sends the unit cube to a set of Lebesgue measure one Lemma
- A subset of ℝⁿ with open supersets of arbitrarily small excess is Lebesgue measurable Lemma
- Agreement of Borel overlap integrals Lemma
- An area-minimal three-sector homoclinic cycle has identity inward holonomy Lemma
- Bounded compact data give an everywhere finite Newtonian potential Lemma
- Compact sets of positive area are not conformally removable Lemma
- Compact subsets of lines and round circles are removable for quasiconformal maps Lemma
- Dyadic coding supplies coin measure and its completed Lebesgue transfer Lemma
- Euclidean balls have positive finite Lebesgue measure Lemma
- For a Lebesgue measurable set and every positive ε there is an open superset whose difference from it has outer measure below ε Lemma
- Fourier uniqueness for continuous functions on the Euclidean torus Lemma
- Lebesgue-point convergence for radial-majorized kernels Lemma
- Local integrability of the Laplace fundamental kernel Lemma
- Monomials form complete orthogonal systems of the Bergman spaces of the disc, the ball and the polydisc Lemma
- Near and far bounds for a Riesz potential Lemma
…and 19 more results.
Dependency tree · two levels
43 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
- John K. Hunter, Measure Theory (UC Davis lecture notes), Proposition 2.21 (standard reference, not scraped)
- E. A. Carlen, Notes on Lebesgue Measure on $\mathbb{R}^n$ and $S^{n-1}$ (Rutgers Math 501), Proposition 1.4 (standard reference, not scraped)