How to put braces over certain parts of matrices?

Question

I want to typeset something like this:

I couldn't find a way to achieve this using array or blkarray. Any help?

Answer 1

You can do this with the nicematrix package. Note that you must compile twice to get the result.

Note also that \OverBrace is a relatively new addition to nicematrix, so if you're using an old installation you'll have to update.

\documentclass{article}
\usepackage{nicematrix}
\begin{document}

\[
m\begin{BNiceArray}{cccc|cc}[right-margin=.4em]
1 & 2 & 3 & 4 & 5 & 6 \\
11 & 12 & 13 & 14 & 15 & 16 \\
\CodeAfter
\SubMatrix[{1-3}{2-6}]
\OverBrace[shorten,yshift=3pt]{1-1}{2-2}{n}
\OverBrace[shorten,yshift=3pt]{1-3}{2-4}{r}
\OverBrace[shorten,yshift=3pt]{1-5}{2-6}{s}
\end{BNiceArray}n
\]

\end{document}

Answer 2

Maybe something like this is what you are looking for:

\begin{array}{c}
  & n & & r & & s & \\
  m\,\big\lbrace & \overbrace{A} & \big[ & \overbrace{P} & \big\vert & \overbrace{Q} & \big] & \big\rbrace\,n
\end{array}

I think it could be made prettier (e.g. fixing some space widths and resizing the variables), but I think the overall idea should be clear.