I would like to post a question here in which I would like to insert a $2 \times 2$ matrix with entries $a$, $b$, $c$, and $d$, respectively, in the positions row 1 col. 1, row 1 col. 2, row 2 col 1, and row 2 col 2. How to?
I would also like to input a general $m \times n$ matrix with entires $a_{ij}$, where $i = 1, 2, \ldots, m$, $j = 1, 2, \ldots, n$.