\begin{definition}[Definition:Matrix Product (Conventional)/Einstein Summation Convention]
Let $\struct {R, +, \circ}$ be a ring.
Let $\mathbf A = \sqbrk a_{m n}$ be an $m \times n$ matrix over $R$.
Let $\mathbf B = \sqbrk b_{n p}$ be an $n \times p$ matrix over $R$.
The matrix product of $\mathbf A$ and $\mathbf B$ can be expressed using the Einstein summation convention as:
Then:
:$c_{i j} := a_{i k} \circ b_{k j}$
The index which appears twice in the expressions on the {{RHS}} is the entry $k$, which is the one summated over.
\end{definition}