# Additive Group of Complex Numbers is Direct Product of Reals with Reals

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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.

Then the direct product $\struct {\R, +} \times \struct {\R, +}$ is isomorphic with $\struct {\C, +}$.

## Proof

We need to show that $\phi: \tuple {x, y} \mapsto x + y i$ is a group isomorphism.

### $\phi$ is a group homomorphism

\(\displaystyle \forall a, b, c, d \in \R: \ \ \) | \(\displaystyle \map \phi {a, b} + \map \phi {c, d}\) | \(=\) | \(\displaystyle \paren {a + b i} + \paren {c + d i}\) | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \paren {a + c} + \paren {b + d} i\) | Complex Addition is Commutative | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \map \phi {a + c, b + d}\) |

$\Box$

### $\phi$ is bijective

We show that $\phi^{-1}: z \mapsto \tuple {\map \Re z, \map \Im z}$ is the inverse of $\phi$.

We have:

- $\map \phi {\map {\phi^{-1} } z} = \map \phi {\map \Re z, \map \Im z} = \map \Re z + i \map \Im z = z$
- $\map {\phi^{-1} } {\map \phi {a, b} } = \map {\phi^{-1} } {a + b i} = \tuple {a, b}$

hence $\phi^{-1}$ is the inverse of $\phi$.

$\Box$

Hence $\phi$ is a group isomorphism, and thus the direct product $\struct {\R, +} \times \struct {\R, +}$ is isomorphic with $\struct {\C, +}$.

$\blacksquare$

## Sources

- 1967: George McCarty:
*Topology: An Introduction with Application to Topological Groups*... (previous) ... (next): Chapter $\text{II}$: Groups: Direct Products