Definition:Matrix Product (Conventional)/Einstein Summation Convention

Definition
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 summation convention as:

Then:
 * $c_{i j} := a_{i k} \circ b_{k j}$

The index which appears twice in the expressions on the is the entry $k$, which is the one summated over.