Let $z = x + i y \in \mathbb{C}$ be a D1207: Complex number such that
| (i) | \begin{equation} z \neq 0 \end{equation} |
| (ii) | \begin{equation} w : = \frac{x - i y}{x^2 + y^2} \end{equation} |
Then
\begin{equation}
z w
= w z
= 1
\end{equation}