Rational Numbers are Countably Infinite/Proof 4

Theorem
The set $\Q$ of rational numbers is countably infinite.

Proof
Let $Q_\pm = \left\{{q \in \Q: \pm q > 0}\right\}$.

For every $q \in Q_+$, there exists at least one pair $\left({m, n}\right) \in \N \times \N$ such that $q = \frac m n$.

Therefore, we can find an injection $i: Q_+ \to \N \times \N$.

By Cartesian Product of Natural Numbers, $\N \times \N$ is countable.

Hence $Q_+$ is countable, by Domain of Injection to Countable Set is Countable.

The map $-: q \mapsto -q$ provides a bijection from $Q_-$ to $Q_+$, hence $Q_-$ is also countable.

Hence $\Q$ is countable.