Alphabeta Math
DefinitionDefinition: AI-adaptedProof: Not applicableaudited 2026-08-29
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The polar surface set function on the unit sphere

Definition

Assume the Axiom of Countable Choice (The Axiom of Countable Choice (ACω)). Fix n≥1 and write Sn−1:={ω∈Rn:∣ω∣=1}. Let Φ:Rn∖{0}⟶(0,∞)×Sn−1,Φ(x)=(∣x∣,x/∣x∣). This map is continuous. If E⊆Sn−1 is Borel, then (0,1]×E is Borel and {rω:ω∈E, 0<r≤1}=Φ−1((0,1]×E) is Borel in Rn∖{0} by A continuous map has Borel preimages of Borel sets, hence Borel in Rn by The Borel sigma-algebra of a subspace is the trace of the ambient Borel sigma-algebra. It is therefore Lebesgue measurable by Assuming countable choice, every Borel subset of Rn is Lebesgue measurable. Define σ(E):=n λn({rω:ω∈E, 0<r≤1}).

The next theorem proves that this is a Borel measure on Sn−1 and that it is exactly the surface measure needed for polar coordinates.

Depends on

Used by

Dependency tree · two levels

26 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