Matrix in vector notation? Let, $\mathbf{A} \in \mathbb{R}^{n \times n}$, $x \in \mathbb{R}^{n}$, and $\mathbf{I}$ be an $n$ by $n$ identity matrix. What does,
$$ \begin{bmatrix}\mathbf{A} \\ \mathbf{I} \end{bmatrix}x,
$$
mean? I see this notation often used in books and no idea what it implies. Thanks!

It denotes a $2n\times 1$ vector with coordinates
$$ [A_1x, \ldots, A_nx, x_1,\ldots,x_n]^{T} $$
Where $A_i$ denotes the $i$-th row of matrix $A$.
