4

I would like to create a matrix with round brackets as displayed on the following picture:enter image description here

I know how to create the following matrix as

\documentclass{article}
\usepackage{amsmath}
\usepackage{stackengine}
\usepackage{mathdots}

\begin{document}

\begin{bmatrix}
a & \hdots & \hdots & c & 0 & \hdots & 0 \\ 
\vdots &  & \iddots & \iddots &  & \iddots  & \\ 
\vdots &  \iddots &  0&  & 0 &  & \\ 
b &\iddots&  & \iddots &  &  & \\ 
0 &   & 0 &  &  &  & \\ 
\vdots & \iddots &  &  &  &  & \\ 
0 &  &  &  &  &  & 
\end{bmatrix}

\end{document}

But I do not know how to create those brackets with those labels. Any help would be much appriciated.

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4 Answers 4

Reset to default
4

Without guessing too much (the only value I guessed is 0.9ex)

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathdots}

\begin{document}

\[
% some preliminaries with local settings
\newcommand{\upperbrace}{%
  \overbrace{\hphantom{\begin{matrix} a & \hdots & \hdots & c \end{matrix}}}%^{k}
}%
\newcommand{\rightbrace}{%
  {\scriptstyle k+1}\,\left\{%
  \vphantom{\begin{matrix} a \\ \vdots \\ \vdots \\ b \end{matrix}}%
  \right.\kern-\nulldelimiterspace
}
% now the real job
\raisebox{\dimexpr-\height-0.9ex}{$\rightbrace$}
\begin{array}[t]{@{}l@{}}
\hphantom{\Bigg\lbrack}\upperbrace^k\hfill\\[-0.5ex]
\begin{bmatrix}
a & \hdots & \hdots & c & 0 & \hdots & 0 \\ 
\vdots &  & \iddots & \iddots &  & \iddots  & \\ 
\vdots &  \iddots &  0&  & 0 &  & \\ 
b &\iddots&  & \iddots &  &  & \\ 
0 &   & 0 &  &  &  & \\ 
\vdots & \iddots &  &  &  &  & \\ 
0 &  &  &  &  &  & 
\end{bmatrix}
\end{array}
\]

\end{document}

enter image description here

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1
  • Thank you very much for your nice reply!! I was going through your code to understand it better and I was thinking if it possible for example to move the upperbrace, as a whole, to the left? Commented May 17, 2021 at 13:44
3

This is a bit of a kludge and it will require fussing with the lengths.

\documentclass[12pt]{article}

% List of packages
\usepackage{amsmath}
\usepackage{amssymb}

\usepackage{mathdots}

\begin{document}

\begin{align*}
&\phantom{0}\overbrace{\phantom{\hspace{6.5em}}}^{\displaystyle k}\\
\raisebox{2.3em}{$k+2 \left\{ \rule[3.5em]{0pt}{0pt} \right.$}
&\begin{bmatrix}
  a & \hdots & \hdots & c & 0 & \hdots & 0 \\ 
  \vdots &  & \iddots & \iddots &  & \iddots  & \\ 
  \vdots &  \iddots &  0&  & 0 &  & \\ 
  b &\iddots&  & \iddots &  &  & \\ 
  0 &   & 0 &  &  &  & \\ 
  \vdots & \iddots &  &  &  &  & \\ 
  0 &  &  &  &  &  & 
\end{bmatrix}
\end{align*}

\end{document}

enter image description here

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2

A propose this solution with pstricks:

\documentclass{article}
\usepackage{mathtools}
\usepackage{blkarray}
\usepackage{stackengine}
\usepackage{mathdots}
\usepackage{pstricks-add}

\begin{document}

\[
\begin{bmatrix}
\pnode[-1ex, 1ex]{A}\rnode[tl]{a}{a} & \hdots & \hdots & \rnode[tr]{C}{c} & 0 & \hdots & 0 \\
\vdots & & \iddots & \iddots & & \iddots & \\
\vdots & \iddots & 0& & 0 & & \\
\pnode[-1ex, 0]{B}\rnode[bl]{b}{b} &\iddots& & \iddots & & & \\
0 & & 0 & & & & \\
\vdots & \iddots & & & & & \\
0 & & & & & &
\end{bmatrix}
\psset{braceWidth=0.6pt, braceWidthInner=4pt, braceWidthOuter=4pt;, nodesepB=-3pt, linejoin=1}
\psbrace[ref=C](C)(a){\simeq}
\psbrace[nodesepA=-2pt, nodesepB=3pt](A)(B){\mathllap{k+1}}
\]

\end{document}

enter image description here

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4
  • Hi, there is a \simeq instead of k in your code. Commented May 14, 2021 at 20:11
  • 1
    Hi, Sebastiano! Actually what was above the top brace was not very easy to read. I hesitated between a rotated \simeq and the number 12. That doesn't change the way to obtain something above that brace anyway Commented May 14, 2021 at 20:28
  • Ah, I have understood. :-)....Effectly it is a strange k that you have taken for \simeq. Commented May 14, 2021 at 20:30
  • 1
    I was not totally convinced it was a \simeq, but it was comparatively close what I was seeing… Commented May 14, 2021 at 20:35
0

Here is what you can do with nicematrix (≥ 6.4 of 2021-11-23):

\documentclass{article}
\usepackage{nicematrix}
\usepackage{tikz}

\begin{document}

$\begin{bNiceMatrix}[first-col]
\Block{4-1}{k\hspace*{3mm}}
& a      & \Ldots  &         & c       & 0 & \Ldots  & 0 \\
& \Vdots &         & \Iddots & \Iddots &   & \Iddots \\
&        &         & 0       &         & 0 \\
& b      & \Iddots &         & \Iddots \\
& 0      &         & 0 \\
& \Vdots & \Iddots \\
& 0 
\CodeAfter
  \SubMatrix{\{}{1-1}{4-1}{.}[xshift=1mm]
  \OverBrace{1-1}{1-4}{k}[shorten,yshift=1mm]
\end{bNiceMatrix}$

\end{document}

You need several compilations (because nicematrix uses PGF/Tikz nodes under the hood).

Output of the above code

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