Imaginary Part as Mapping is Endomorphism for Complex Addition

Theorem
Let $\struct {\C, +}$ be the additive group of complex numbers.

Let $\struct {\R, +}$ be the additive group of real numbers.

Let $f: \C \to \R$ be the mapping from the complex numbers to the real numbers defined as:
 * $\forall z \in \C: \map f z = \map \Im z$

where $\map \Im z$ denotes the imaginary part of $z$.

Then $f: \struct {\C, +} \to \struct {\R, +}$ is a group epimorphism.

Its kernel is the set:
 * $\map \ker f = \R$

of (wholly) real numbers.

Proof
From Imaginary Part as Mapping is Surjection, $f$ is a surjection.

Let $z_1, z_2 \in \C$.

Let $z_1 = x_1 + i y_1, z_2 = x_2 + i y_2$.

Then:

So $f$ is a group homomorphism.

Thus $f$ is a surjective group homomorphism and therefore by definition a group epimorphism.

Finally:
 * $\forall y \in \R: \map \Im {x + 0 i} = 0 i = 0$

It follows from Complex Addition Identity is Zero that:
 * $\map \ker f = \set {x: x \in \R} = \R$

Also see

 * Real Part as Mapping is Endomorphism for Complex Addition