# Real Part as Mapping is Surjection

## Theorem

Let $f: \C \to \R$ be the projection 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 $f$ is a surjection.

## Proof

Let $x \in \R$ be a real number.

Let $y \in \R$ be an arbitrary real number.

Let $z \in \C$ be the complex number defined as:

$z = x + i y$

Then we have:

$\map \Re z = x$

That is:

$\exists z \in \C: \map f z = x$

The result follows by definition of surjection.

$\blacksquare$