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LemmaStatement: Literature-sourcedProof: AI-adaptedprecheck passjudge pass (gpt-5.6-terra)audited 2026-09-14
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Bochner integral norm inequality

Statement

If f:Ω→X is Bochner integrable, then for every measurable E,

∥∫Ef dμ∥≤∫E∥f∥ dμ.

Facts & Assumptions

[L1]

For a strongly measurable function, Bochner integrability is equivalent to finite integrability of its norm. Every Bochner-integrable function has a defining integrable-simple approximation converging in L1 (Bochner integrability criterion).

[L2]

The inequality holds for integrable Banach-valued simple functions (The Banach-valued simple integral is well defined).

Proof

technique · direct

Given: A Bochner-integrable f and a measurable set E.

1.1givenL1choose

Restrict a defining simple approximation to E. [given, L1, choose] Choose integrable simple sn with ∫∥f−sn∥→0. Replacing each by 1Esn shows from [L1] that 1Ef is Bochner integrable and that its integral is the norm limit of ∫Esn.

2.1L2L3step 1.1

Bound each simple integral. [L2, L3, step 1.1] The triangle inequality and [L2] give ∥∫Esn∥≤∫E∥sn∥. Since ∥sn∥≤∥f∥+∥sn−f∥, [L3] yields ∥∫Esn∥≤∫E∥f∥+∫E∥sn−f∥.

3.1step 1.1step 2.1∎

Pass to the limit and conclude. [step 1.1, step 2.1] Let n→∞ in step 2.1. Norm continuity and step 1.1 identify the left limit with ∥∫Ef∥, while the last term tends to zero. This proves the inequality, including E=∅ and f=0.

Depends on

Used by

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Sources