michaeld wrote:

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In contrast it makes no sense to talk of a space being 'closed'. You
can only talk about a space being closed in (blah) - another space. If
X is a topological space then a subspace A is closed in X iff the
complement X - A is open in A.

Sorry, that should of course say 'open in X' not 'open in A'. A is
closed in X iff A^c := X-A is open in X.

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