Epimorphism from Integers to Cyclic Group

Theorem
Let $\left \langle {a} \right \rangle = \left({G, \circ}\right)$ be a cyclic group.

Let $f: \Z \to G$ be a mapping defined as:

$\forall n \in \Z: f \left({n}\right) = a^n$.

Then $f$ is an epimorphism from $\left({\Z, +}\right)$ onto $\left \langle {a} \right \rangle$.

Proof
By induction, $\forall n \in \N: a^n \in \left \langle {a} \right \rangle$.

Hence by the Index Law for Monoids, $\forall n \in \Z: a^n \in \left \langle {a} \right \rangle$.

Also, by Index Law for Sum of Indices, $f$ is a homomorphism from $\left({\Z, +}\right)$ into $\left({G, \circ}\right)$.

Its codomain $f \left({\Z}\right)$ is therefore a subgroup of $\left \langle {a} \right \rangle$ containing $a$ by Homomorphism Preserves Subsemigroups.

So $f \left({\Z}\right) = \left \langle {a} \right \rangle$ because $\left \langle {a} \right \rangle$ is the smallest subgroup of $G$ containing $a$ by Generator of a Group.

Therefore $f$ is an epimorphism from $\left({\Z, +}\right)$ onto $\left({G, \circ}\right)$.