How to compute the product of two matrices?

    

$\text{row }i \rightarrow\left( \begin{matrix} \begin{matrix}a_{11} & a_{12} & \... ...\ b_{2p} \\ \dots \\ b_{ip} \\ \dots \\ b_{np} \end{matrix}\end{matrix}\right)$

$\begin{matrix}\qquad & \qquad & \qquad & \qquad & \qquad & \qquad & \qquad & \q... ...qquad \begin{matrix}\uparrow \\ \text{column nb } j \end{matrix} \end{matrix}$

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