Labelling a matrix using a brace in latex

Question

Hey, I want to add two braces as labels for a matrix. I am able to add the top brace label "r". But when I add the bottom "m - r" label, it becomes branched. Here's my code in Latex.

\[R = 
\raisebox{0.5em}{$r \left\{ \rule[2em]{0pt}{0pt} \right. $} 
\raisebox{-2em}{$m - r \left\{ \rule[0.5em]{0pt}{0pt} \right. $}
\left[
\begin{array}{ccc|cccc}
  1 & & & b_{1, r + 1} & b_{1, r + 2} & \ldots & b_{1n}\\
  & \ddots & & \ldots & \ldots & \ddots & \ldots \\
  & & 1 & b_{r, r + 1} & b_{r, r + 2} & \ldots & b_{rn} \\
  \hline
  & O & & & O & & 
\end{array}
\right]
\]

Answer 1

Here is a solution with nicematrix. Note the use of \cdots and \vdots. Also the O in the bottom right quadrant is centered in the block, not in its own column.

\documentclass{article}

\usepackage{nicematrix}

\begin{document}
\[R = 
\begin{bNiceArray}{ccc|cccc}[first-col, margin]
\Block{3-1}{\scriptstyle {r\hspace{2mm}}} & 1 & & & b_{1, r + 1} & b_{1, r + 2} & \cdots & b_{1n}\\
 & & \ddots & & \vdots & \vdots & \ddots & \vdots \\
 & & & 1 & b_{r, r + 1} & b_{r, r + 2} & \cdots & b_{rn} \\ \hline
\scriptstyle {m-r\hspace{2mm}} & & O & & \Block{1-4}{O} & & & 
\CodeAfter
  \SubMatrix\{{1-1}{3-7}.[left-xshift=3mm]
  \SubMatrix\{{4-1}{4-7}.[left-xshift=3mm]
\end{bNiceArray}.
\]
\end{document}

As an alternative, you can use the tikz libraries decorations.pathreplacing and calligraphy together with nicematrix to draw the braces. This has the advantage of giving the braces the same thickness regardless of their length, but nicematrix ignores the braces and their labels when creating space for the matrix, so additional space must be placed manually. I used \hspace{7mm} in the code below.

Also (optional), shorten <= does not work with the calligraphic brace, so the endpoints should use yshift to shorten the braces.

\documentclass{article}

\usepackage{nicematrix, tikz}
\usetikzlibrary{decorations.pathreplacing, calligraphy}
\tikzset{mybrace/.style={thick, decorate, decoration={calligraphic brace, amplitude=2pt, raise=2mm}}}

\begin{document}
\[R = \hspace{7mm}
\begin{bNiceArray}{ccc|cccc}[margin]
1 & & & b_{1, r + 1} & b_{1, r + 2} & \cdots & b_{1n}\\
  & \ddots & & \vdots & \vdots & \ddots & \vdots \\
  & & 1 & b_{r, r + 1} & b_{r, r + 2} & \cdots & b_{rn} \\ \hline
  & O & & \Block{1-4}{O} & & & 
\CodeAfter
  \tikz{
    \draw[mybrace]([yshift=1pt]4-|1)--node[left=2mm]{$\scriptstyle r$}([yshift=-1pt]1-|1);
    \draw[mybrace]([yshift=1pt]5-|1)--node[left=2mm]{$\scriptstyle m-r$}([yshift=-1pt]4-|1);
  }
\end{bNiceArray}.
\]
\end{document}