Quotient Theorem for Group Epimorphisms

Theorem
Let $\phi: \left({G, \oplus}\right) \to \left({H, \odot}\right)$ be a group epimorphism.

Let $e_G$ and $e_H$ be the identities of $G$ and $H$ respectively.

Let $K = \ker \left({\phi}\right)$.

There is one and only one group isomorphism $\psi: G / K \to H$ satisfying:


 * $\psi \circ q_K = \phi$

The epimorphism $\phi$ is an isomorphism iff $K = \left\{{e_G}\right\}$.

Proof
Let $\mathcal R_\phi$ be the equivalence on $G$ defined by $\phi$.

Thus:
 * $K = \left[\!\left[{e_G}\right]\!\right]_{\mathcal R_\phi}$

From the Quotient Theorem for Epimorphisms:
 * $\mathcal R_\phi$ is compatible with $\oplus$

Thus from Kernel is Normal Subgroup of Domain:
 * $K \lhd G$

From Congruence Relation induces Normal Subgroup, $\mathcal R_\phi$ is the equivalence defined by $K$.

Thus, again by Quotient Theorem for Epimorphisms, there is a unique epimorphism $\psi: G / K \to H$ satisfying $\psi \circ q_K = \phi$.

Now let $\phi$ be an isomorphism.

Then $K = \left\{{e_G}\right\}$ as $\phi$ is injective.

Conversely, if $K = \left\{{e_G}\right\}$ and $\phi \left({x}\right) = \phi \left({y}\right)$, then:
 * $x \mathcal R_K y$ as $\mathcal R_\phi = \mathcal R_K$

from Congruence Modulo Subgroup is Equivalence Relation.

Thus by Congruence Class Modulo Subgroup is Coset:
 * $x \oplus y^{-1} \in K$

Hence:
 * $x \oplus y^{-1} = e_G$

and so:
 * $x = y$

Thus $\phi$ is injective, and an injective epimorphism is a isomorphism.

Also known as
Some sources call this the Factor Theorem for Group Epimorphisms.