Proposition 2
Tag 0C2H, permanent. Cite it as [Bourbaki, Tag 0C2H].
General Topology, chapter IX, § 2. Metric spaces and metrizable spaces
The statements and are equivalent.
For expresses the fact that the ball meets whatever the value of ; and this is equivalent to .
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.
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Cites
Nothing.