# Real Part as Mapping is Endomorphism for Complex Addition

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## 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:

- $\forall z \in \C: \map f z = \map \Re z$

where $\map \Re z$ denotes the real part of $z$.

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

Its kernel is the set:

- $\map \ker f = \set {i x: x \in \R}$

## Proof

From Real 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:

\(\ds \map f {z_1 + z_2}\) | \(=\) | \(\ds \map \Re {z_1 + z_2}\) | Definition of $f$ | |||||||||||

\(\ds \) | \(=\) | \(\ds \map \Re {x_1 + i y_1 + x_2 + i y_2}\) | Definition of $z_1$ and $z_2$ | |||||||||||

\(\ds \) | \(=\) | \(\ds x_1 + x_2\) | Definition of Real Part | |||||||||||

\(\ds \) | \(=\) | \(\ds \map \Re {z_1} + \map \Re {z_2}\) | Definition of Real Part | |||||||||||

\(\ds \) | \(=\) | \(\ds \map f {z_1} + \map f {z_2}\) | Definition of $f$ |

So $f$ is a group homomorphism.

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

Finally:

- $\forall y \in \R: \map \Re {0 + i y} = 0$

It follows from Complex Addition Identity is Zero that:

- $\map \ker f = \set {i x: x \in \R}$

$\blacksquare$

## Also see

## Sources

- 1965: Seth Warner:
*Modern Algebra*... (previous) ... (next): Chapter $\text {II}$: New Structures from Old: $\S 12$: Homomorphisms: Example $12.3$ - 1967: George McCarty:
*Topology: An Introduction with Application to Topological Groups*... (previous) ... (next): Chapter $\text{II}$: Groups: Direct Products