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单词 CorollaryOfBanachAlaogluTheorem
释义

corollary of Banach-Alaoglu theorem


Corollary.

A Banach spaceMathworldPlanetmath H is isometrically isomorphic to a closed subspace of C(X) for a compactPlanetmathPlanetmath Hausdorff space X.

Proof.

Let X be the unit ball (*) of *. By the Banach-Alaoglu theorem it is compact in the weak-* topologyMathworldPlanetmath. Define the map Φ:C(X) by (Φf)(φ)=φ(f). This is linear and we have for f:

Φ(f)=supφ(*)|Φ(f)(φ)|=supφ(*)|φ(f)|supφ(*)φff

With the Hahn-Banach theoremMathworldPlanetmath it follows that there is a φ(*) such that φ(f)=f. Thus Φ(f)=f and Φ is an isometric isomorphism, as required.∎

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更新时间:2025/5/4 5:52:34