Epimorphism from Integers to Cyclic Group
Theorem
Let $\gen a = \struct {G, \circ}$ be a cyclic group.
Let $f: \Z \to G$ be a mapping defined as:
$\forall n \in \Z: \map f n = a^n$.
Then $f$ is a (group) epimorphism from $\struct {\Z, +}$ onto $\gen a$.
Proof
By Powers of Element form Subgroup:
- $\forall n \in \N: a^n \in \gen a$
Hence by the Index Law for Monoids: Negative Index:
- $\forall n \in \Z: a^n \in \gen a$
Also, by Index Law for Monoids: Sum of Indices, $f$ is a homomorphism from $\struct {\Z, +}$ into $\struct {G, \circ}$.
By Homomorphism Preserves Subsemigroups, its codomain $f \sqbrk \Z$ is therefore a subgroup of $\gen a$ containing $a$.
By Existence of Unique Subgroup Generated by Subset: Singleton Generator, $\gen a$ is the smallest subgroup of $G$ containing $a$.
So:
- $f \sqbrk \Z = \gen a$
Therefore $f$ is an epimorphism from $\struct {\Z, +}$ onto $\struct {G, \circ}$.
$\blacksquare$
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {IV}$: Rings and Fields: $25$. Cyclic Groups and Lagrange's Theorem: Theorem $25.1$