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单词 ACompleteSubspaceOfAMetricSpaceIsClosed
释义

a complete subspace of a metric space is closed


Let X be a metric space, and let Y be a complete subspaceMathworldPlanetmath of X. Then Y is closed.

Proof

Let xY¯ be a point in the closurePlanetmathPlanetmath of Y. Then by the definition of closure, from each ball B(x,1n) centered in x, we can select a point ynY. This is clearly a Cauchy sequencePlanetmathPlanetmath in Y, and its limit is x, hence by the completeness of Y, xY and thus Y=Y¯.

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