Countable Discrete Space is Second-Countable

Theorem
Let $T = \left({S, \tau}\right)$ be a countable discrete topological space.

Then $T$ is second-countable.

Proof
From Basis for Discrete Topology, the set:
 * $\mathcal B := \left\{{\left\{{x}\right\}: x \in S}\right\}$

is a basis for $T$.

There is a trivial one-to-one correspondence $\phi: S \leftrightarrow \mathcal B$ between $S$ and $\mathcal B$:
 * $\forall x \in S: \phi \left({x}\right) = \left\{{x}\right\}$

Let $S$ be countable.

Then $\mathcal B$ is also countable by definition of countability.

So we have that $T$ has a countable basis, and so is second-countable by definition.

Also see

 * Uncountable Discrete Space is not Second-Countable