Matrix Multiplication is Associative

Theorem
Let $R$ be a ring.

Matrix multiplication (conventional) is associative.

Proof
Let $\mathbf A = \sqbrk a_{m n}, \mathbf B = \sqbrk b_{n p}, \mathbf C = \sqbrk c_{p q}$ be matrices.

From inspection of the subscripts, we can see that both $\paren {\mathbf A \mathbf B} \mathbf C$ and $\mathbf A \paren {\mathbf B \mathbf C}$ are defined:

$\mathbf A$ has $n$ columns and $\mathbf B$ has $n$ rows, while $\mathbf B$ has $p$ columns and $\mathbf C$ has $p$ rows.

Consider $\paren {\mathbf A \mathbf B} \mathbf C$.

Let $\mathbf R = \sqbrk r_{m p} = \mathbf A \mathbf B, \mathbf S = \sqbrk s_{m q} = \mathbf A \paren {\mathbf B \mathbf C}$.

Then:

Now consider $\mathbf A \paren {\mathbf B \mathbf C}$.

Let $\mathbf R = \sqbrk r_{n q} = \mathbf B \mathbf C, \mathbf S = \sqbrk s_{m q} = \mathbf A \paren {\mathbf B \mathbf C}$.

Then:

Using ring axiom $(M1)$ (associativity of $\circ$):


 * $\displaystyle s_{i j} = \sum_{k \mathop = 1}^p \sum_{l \mathop = 1}^n \paren {a_{i l} \circ b_{l k} } \circ c_{k j} = \sum_{l \mathop = 1}^n \sum_{k \mathop = 1}^p a_{i l} \circ \paren {b_{l k} \circ c_{k j} } = s'_{i j}$

It is concluded that:
 * $\paren {\mathbf A \mathbf B} \mathbf C = \mathbf A \paren {\mathbf B \mathbf C}$