Separable Discrete Space is Countable
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Theorem
Let $T = \struct {S, \tau}$ be a discrete topological space.
Let $T$ be separable.
Then $S$ is countable.
Proof 1
Let $T$ be separable.
By Space is Separable iff Density not greater than Aleph Zero:
- $\map d T \le \aleph_0$
where:
- $\map d T$ denotes the density of $T$
- $\aleph$ denotes the aleph mapping.
By definition of density:
- $\exists A \subseteq S: A$ is everywhere dense and $\map d T = \card A$
where $\card A$ denotes the cardinality of $A$.
By definition of everywhere dense set:
- $A^- = S$
where $A^-$ denotes the closure of $A$.
By Set in Discrete Topology is Clopen:
- $A$ is closed
Then by Set is Closed iff Equals Topological Closure:
- $A^- = A$
Thus by Countable iff Cardinality not greater than Aleph Zero:
- $S$ is countable.
$\blacksquare$
Proof 2
Let $T$ be separable.
Aiming for a contradiction, suppose $S$ is uncountable.
Then by Uncountable Discrete Space is not Separable, $T$ is not separable.
Hence the result by Proof by Contradiction.
$\blacksquare$