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 $$\left({G, \circ}\right)$$.