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单词 CompactSubspaceOfAHausdorffSpaceIsClosed
释义

compact subspace of a Hausdorff space is closed


Let X be a Hausdorff space, and Y be a compactPlanetmathPlanetmath subspaceMathworldPlanetmathPlanetmath of X. We prove that XY is open, by finding for every point xXY a neighborhoodMathworldPlanetmathPlanetmath Ux disjoint from Y.

Let yY. xy, so by the definition of a Hausdorff space, there exist open neighborhoods Ux(y) of x and Vx(y) of y such that Ux(y)Vx(y)=. Clearly

YyYVx(y)

but since Y is compact, we can select from these a finite subcover of Y

YVx(y1)Vx(yn)

Now for every yY there exists k1n such that yVx(yk). Since Ux(yk) and Vx(yk) are disjoint, yUx(yk), therefore neither is it in the intersectionMathworldPlanetmath

Ux=j=1nUx(yj)

A finite intersection of open sets is open, hence Ux is a neighborhood of x disjoint from Y.

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更新时间:2025/5/5 2:57:46