Alphabeta Math
TheoremStatement: Literature-sourcedProof: AI-adaptedPipeline-generatedverified 2026-09-23 (gpt-6-sol)
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C_c(X) is dense in L^p(mu) for a Radon measure

Statement

Assume Dependent Choice (The axiom of dependent choice: a relation in which every element is related to something admits an N-indexed chain). If μ is a Radon measure on an LCH space X and 1≤p<∞, then Cc(X) is dense in Lp(μ).

Facts & Assumptions

Given: DC, μ is Radon and 1≤p<∞.

[L1]
[L2]

Under the stated DC premise, LCH cutoffs exist between compact and open sets (LCH Urysohn cutoff). Proof 2.1 is the exact use of DC.

Proof

technique · direct
1.1L1

It suffices by [L1] to approximate 1E when μ(E)<∞. [L1, choose] Given η>0, outer regularity gives open U⊇E with μ(U∖E)<ηp/2, and inner regularity of U gives compact K⊆U with μ(U∖K)<ηp/2.

2.1step 1.1L2

Choose f∈Cc(X) with 1K≤f≤1U by [L2]. [step 1.1, L2] Then ∣f−1E∣≤1U∖K+1U∖E. Since both indicators take only the values zero and one and their union is U∖(K∩E), while 0≤f,1E≤1, the error is bounded by the indicator of that union. Hence ∥f−1E∥pp≤μ(U∖K)+μ(U∖E)<ηp. Thus ∥f−1E∥p<η.

3.1step 2.1L1∎

Approximate the finitely many indicator terms of a simple function separately and sum the resulting Cc functions. Then use [L1] and the triangle inequality.

Depends on

Used by

Dependency tree · two levels

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Sources