Solution
= Solution
Apply row-column <matrix multiplication> twice:
$$
\boxed{(ABC)_{ij}=a_{ik}b_{k\ell}c_{\ell j}.}
$$
The indices $i,j$ are free, while $k,\ell$ are summed.
= Solution
Apply row-column <matrix multiplication> twice:
$$
\boxed{(ABC)_{ij}=a_{ik}b_{k\ell}c_{\ell j}.}
$$
The indices $i,j$ are free, while $k,\ell$ are summed.