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单词 UrysohnExtensionTheorem
释义

Urysohn extension theorem


Let X be a topological spaceMathworldPlanetmath, and C(X) and C*(X) the rings of continuous functions and boundedPlanetmathPlanetmathPlanetmathPlanetmath continuous functionsMathworldPlanetmathPlanetmath respectively.

Urysohn Extension Theorem. A subset AX is C*-embedded (http://planetmath.org/CEmbedding) if and only if any two completely separated sets in A are completely separated in X as well.

Remarks.

  • Suppose that X is a metric space and A is closed in X. If S,T are completely separated sets in A, then they are contained in disjoint zero setsPlanetmathPlanetmath S and T in A. Since S and T are closed in A, and A is closed in X, S and T are closed in X. Since X is a metric space, S and T are zero sets in X. Since SS and TT and ST are disjoint, S and T are completely separated in X as well. By Urysohn Extension Theorem, any bounded continuous function defined on A can be extended to a continuous function on X, which is the statement of the metric space version of the Tietze extension theorem.

  • However, the above argumentMathworldPlanetmath does not generalize to normal spacesMathworldPlanetmath, so can not be used to prove the generalized version of the Tietze extension theorem. Urysohn’s lemma is required to prove this more general result.

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更新时间:2025/5/4 19:04:48