Alphabeta Math
TheoremStatement: AI-adaptedProof: AI-adaptedPipeline-generatedaudited 2026-09-07
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The Cantor set has dimension log 2 / log 3 and critical measure one

Statement

Assume the Axiom of Countable Choice. For the middle-thirds Cantor set C and s=log⁡2/log⁡3,

Hs(C)=1,dim⁡HC=s.

These values use the unnormalised diameter-power convention.

Facts & Assumptions

Given: The objects, conventions, and hypotheses in the statement above.

[F1]

Under the standing Countable Choice hypothesis, for the Cantor probability measure, every nonempty bounded set U has μc∗(U)≤(diam⁡U)s at s=log⁡2/log⁡3. The sharp interval bound for Cantor measure

[F2]

Under the standing Countable Choice hypothesis, a finite Borel measure with outer mass on A positive and diameter bound C0rs gives Hs(A)≥μ∗(A)/C0. The mass distribution principle

[F3]

Finite positive Hausdorff measure at s forces dimension s. Hausdorff dimension is the unique critical exponent

[F5]

Positive real powers obey the multiplication and power-of-power laws. The exponent, product, quotient, and iterated-power laws for positive real bases and real exponents

[F6]

Under the standing Countable Choice hypothesis, μc is a probability measure concentrated on C. The Cantor measure is a singular atomless probability measure concentrated on the Cantor set

[F7]

Under Countable Choice, μc=μFc, where the extended Cantor function Fc is nondecreasing and right-continuous on R. The Cantor measure

[F8]

Under Countable Choice, the Lebesgue–Stieltjes measure μF of a nondecreasing right-continuous real function F is a Borel measure on R. Assuming countable choice, a nondecreasing right-continuous function defines a Borel measure on R

Proof

1.1F4F5

The 2m length-3−m basic intervals cover C and have total s-cost 2m(3−m)s=1. At each positive scale take m sufficiently large. Thus Hs(C)≤1, including the level-zero cover at its own scale.

2.1F1F2F3F6F7F8step 1.1∎

The defining function in F7 satisfies F8, so μc is a Borel measure; F6 makes it finite with total mass one and μc(R∖C)=0. Every Borel superset B of C has μc(R∖B)=0 by monotonicity, hence μc(B)=1. Thus the Borel-hull outer measure in F2 satisfies μc∗(C)=1. F1 supplies its diameter bound with constant one, in particular for every nonempty set of diameter less than r0=1. F2 gives Hs(C)≥1. Together with step 1.1 this gives finite positive measure one, so F3 gives dim⁡HC=s.

Depends on

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