Let $+$ be the D637: Natural number addition operation.

Let $\cdot$ be the D638: Natural number multiplication operation.

The

**integer multiplication operation**is the D554: Binary operation \begin{equation} \mathbb{Z} \times \mathbb{Z} \to \mathbb{Z}, \quad [(a, b)] [(c, d)] = [(a c + b d, a d + b c)] \end{equation}