Definition:Multiplication/Complex Numbers

Definition
The multiplication operation in the domain of complex numbers $\C$ is written $\times$.

Let $z = a + i b, w = c + i d$ where $a, b, c, d \in \R$.

Then $z \times w$ is defined as $\left({a + i b}\right) \times \left({c + i d}\right) = \left({ac - bd}\right) + i \left({ad + bc}\right)$.

This follows by the facts that:
 * the real numbers form a field and thus multiplication distributes over addition;
 * the entity $i$ is such that $i^2 = -1$.

Also see

 * Complex Multiplication is Commutative
 * Complex Multiplication is Associative