Surjection/Examples/Negative Function on Integers

Example of Surjection
Let $f: \Z \to \Z$ be the mapping defined on the set of integers as:
 * $\forall x \in \Z: \map f x = -x$

Then $f$ is a surjection.

Proof
For $f$ to be a surjection, it is necessary that:
 * $\forall y \in \Z: \exists x \in \Z: -x = y$

This is the case by setting $ x = -y$, which is an integer since $y$ is an integer.

Thus $f$ is a surjection by definition.