Alphabeta Math
False statementConstruction: AI-adaptedVerification: AI-adaptedPipeline-generatedaudited 2026-09-22
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Compact Haar measure is bi-invariant

Statement

Assume the Axiom of Choice. False: normalized Haar measure on a compact Lie group is only left invariant, not right invariant.

Facts & Assumptions

Given: Assume the Axiom of Choice, a compact Lie group G with normalized Haar measure μ.

[A1]

The Axiom of Choice is The Axiom of Choice; it enters through [L1].

[L1]

Every compact Hausdorff group has a unique left Haar probability measure, which is right invariant and inversion invariant (Normalized Haar probability on a compact group). A left Haar measure is nonzero, left invariant, finite on compact sets, outer regular on Borel sets and inner regular on open sets (Left Haar integral and left Haar measure). In particular every regular left-invariant Borel probability on a compact Lie group is a left Haar probability. The compact Lie group specialization is Normalized Haar measure on a compact Lie group.

[L2]

A finite Radon measure μ is right invariant when μ(Eh)=μ(E) for all Borel E and all hG (Left, right, and bi-invariant Borel measures).

Refutation

technique · direct
1.1

For fixed hG define μh(E):=μ(Eh); since right translation is a homeomorphism and μ is a regular Borel probability, μh is again a regular Borel probability, and it is left invariant: μh(gE)=μ(gEh)=μ(Eh)=μh(E) for all g, because μ is left invariant.

L1L2given
2.1

The measure μh is a regular left-invariant Borel probability by step 1.1, so the uniqueness among left-invariant regular Borel probabilities in [L1] gives μh=μ for every h. Thus μ(Eh)=μ(E) for all Borel E and h: normalized Haar measure on a compact Lie group is right invariant as well as left invariant.

L1step 1.1
3.1

Hence the statement of this item is false: the correct conclusion is bi-invariance, and the uniqueness argument above shows that no example of a compact Lie group with a merely left-invariant normalized Haar measure exists. The trivial group is included: its sole probability measure is the point mass, invariant on both sides.

A1L1step 2.1

Depends on

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