Is there a notation for the question mark place?
A and B are standard real general matrices.
AB means standard matrix multiplication of A and B.
$$
\begin{pmatrix} AB & AB \\ AB & AB \end{pmatrix} = A \, ? \, \begin{pmatrix} B & B \\ B & B \end{pmatrix}
$$
Best Answer
Note that blockwise multiplication by $A$ is the same as multiplying by the block-diagonal matrix $$\begin{bmatrix} A & 0 \\ 0 & A \end{bmatrix}.$$ In the past, I've used various notations to refer to the above, such as $[[A]]$ and Diag($A$), and just made sure to define my notation before using it.
That said, I'm also curious whether a standard notation exists.