Closed Sets of Fortissimo Space
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Theorem
Let $T = \struct {S, \tau_p}$ be a Fortissimo space.
Then $H \subseteq S$ is closed in $T$ if and only if:
- $p \in H$
or
- $H$ is countable
or both.
Proof
By definition of a Fortissimo space, $U \subseteq S$ is open in $T$ if and only if:
- $p \in \relcomp S U$
or
- $\relcomp S U$ is countable
or both.
The result follows from the definition of closed set.
$\blacksquare$