Complex Numbers form Algebra

Theorem
The set of complex numbers $\C$ forms an algebra over the field of real numbers.

This algebra is:
 * $(1): \quad$ An associative algebra.
 * $(2): \quad$ A commutative algebra.
 * $(3): \quad$ A normed division algebra.
 * $(4): \quad$ A nicely normed $*$-algebra.

However, $\C$ is not a real algebra.

Proof
The complex numbers $\C$ are formed by the Cayley-Dickson Construction from the real numbers $\R$.

From Real Numbers form Algebra, we have that $\R$ forms:
 * $(1): \quad$ An associative algebra.
 * $(2): \quad$ A commutative algebra.
 * $(3): \quad$ A normed division algebra.
 * $(4): \quad$ A nicely normed $*$-algebra whose $*$ operator is the identity mapping.
 * $(5): \quad$ A real $*$-algebra.

From Cayley-Dickson Construction forms Star-Algebra, $\C$ is a $*$-algebra.

From Cayley-Dickson Construction from Nicely Normed Algebra is Nicely Normed, $\C$ is a nicely normed $*$-algebra.

From Cayley-Dickson Construction from Real Algebra is Commutative, $\C$ is a commutative algebra.

From Cayley-Dickson Construction from Commutative Associative Algebra is Associative, $\C$ is an associative algebra.

However, from Algebra from Cayley-Dickson Construction Never Real, $\C$ is not a real algebra.

Proof of Normed Division Algebra
Consider the element $\left({1, 0}\right)$ of $\R^2$.

We have:

As $\times$ has already been shown to be commutative, it follows that $\left({1, 0}\right) \times \left({x_1, x_2}\right) = \left({x_1, x_2}\right)$.

So $\left({1, 0}\right) \in \R^2$ functions as a unit.

That is, $\left({\R^2, \times}\right)$ is a unitary algebra.

We define a norm on $\left({\R^2, \times}\right)$ by:
 * $\forall \mathbf a = \left({a_1, a_2}\right) \in \R^2: \left \Vert {\mathbf a} \right \Vert = \sqrt {a_1^2 + a_2^2}$

This is a norm because:


 * $(1): \quad \left \Vert \mathbf x \right \Vert = 0 \iff \mathbf x = \mathbf 0$
 * $(2): \quad \left \Vert \lambda \mathbf x \right \Vert = \left \vert \lambda \right \vert \left \Vert x \right \Vert$
 * $(3): \quad \left \Vert x - y \right \Vert \le \left \Vert x - z \right \Vert + \left \Vert z - y \right \Vert$

It also follows that:
 * $\left \Vert x \times y \right \Vert = \left \vert x \times y \right \vert = \left \vert x \right \vert \times \left \vert y \right \vert = \left \Vert x \right \Vert \times \left \Vert y \right \Vert$

and so $\left({\R^2, \times}\right)$ is a normed division algebra.