Let $x_1, \ldots, x_N \in \mathbb{R}$ and $y_1, \ldots, y_M \in \mathbb{R}$ each be a D993: Real number.

Then \begin{equation} \left( \sum_{n = 1}^N x_n \right) \left( \sum_{m = 1}^M y_m \right) = \sum_{n = 1}^N \sum_{m = 1}^M x_n y_m \end{equation}