As we all know, Baire Category theorem has two equivalent forms
- $X$ is a complete metric space, then the countable intersection of dense open sets is nonempty.
- $X$ is a complete metric space, $X$ is a second category set.
Two forms are equivalent if $X$ is complete. If $X$ is a general metric space, are they still equivalent?
Best Answer
The general equivalence is just (for any topological space, not just metrisable ones)
E.g. see Willard (25.2). Just note that a set of nowhere dense iff its complement contains a dense open set. A Baire space is a space where any countable intersection of dense open subsets is dense.
Then there is a separate theorem that a complete metric space is a Baire space, so that the above general equivalence applies to them as well.