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Every translation-invariant measure on the Borel sets of is a nonnegative multiple of Lebesgue measure
Statement
Assume the Axiom of Countable Choice. Every translation-invariant measure on the Borel sets of is a nonnegative multiple of Lebesgue measure.
Facts & Assumptions
Given: The Axiom of Countable Choice and counting measure on , restricted to the Borel sigma-algebra.
Assuming countable choice, a translation-invariant measure on the Borel sets of giving the unit cube measure one is the restriction of Lebesgue measure (A translation-invariant measure on the Borel sets of giving the unit cube measure one is the restriction of Lebesgue measure).
The counting set function on is the map sending a finite subset to its cardinality and an infinite subset to (Counting measure on an arbitrary set).
For every set , the counting set function is a measure on (Counting measure is a measure).
Assuming countable choice, a box in with parameters is Lebesgue measurable of measure (A box in with parameters is Lebesgue measurable of measure , whichever of its faces are included).
Refutation
By [F1] and [F2], counting measure restricted to the Borel sets of is a measure. It is translation invariant because for every real the map is a bijection of , so a Borel set and its translate have the same finite cardinality or are both infinite.
It gives the singleton the value and the unit interval the value , while [L2] gives Lebesgue measure for and for ; so no nonnegative scalar multiple of Lebesgue measure equals counting measure.
This does not contradict [L1], because [L1] fixes the value of the unit cube and thereby forces finiteness on bounded sets, which the false statement omits.
Depends on
- A translation-invariant measure on the Borel sets of $\mathbb{R}^n$ giving the unit cube measure one is the restriction of Lebesgue measure
- Counting measure on an arbitrary set
- Counting measure is a measure
- A box in $\mathbb{R}^n$ with parameters $a_i\le b_i$ is Lebesgue measurable of measure $\prod_{i<n}(b_i-a_i)$, whichever of its faces are included
- The Borel sigma-algebra of a topological space
Used by
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Sources
- T. Tao, An Introduction to Measure Theory (GSM 126), Exercise 1.2.23 (standard reference, not scraped)