Underbrace in a matrix

Question

I want to display the structure of a matrix as shown in the image below:

enter image description here

Can this be done using pmatrix and \underbrace?

My attempt at combining pmatrix and \underbrace results in a compile error.

\documentclass{standalone}

\usepackage{amsmath}

\begin{document}
$
\begin{pmatrix}
%1 & \underbrace{1 & \cdots & 1}_{k} \\ % compile error!
1 & 1 & \cdots & 1 \\
1 & 1 & 1 & 1
\end{pmatrix}
$
\end{document}

The answers in the following questions may help: http://tex.stackexchange.com/q/109054/18228, http://tex.stackexchange.com/q/102460/18228. In general, tikzmark would be very useful — Herr K., Nov 19 '13 at 00:41

score 25 · Accepted Answer · edited Feb 21 '17 at 21:08

25

A combination of \smash[b] should work:

\documentclass{standalone}
\usepackage{amsmath}

\newcommand{\block}[1]{
  \underbrace{\begin{matrix}1 & \cdots & 1\end{matrix}}_{#1}
}

\begin{document}
$
\underbrace{
  \begin{pmatrix}
  1 & \smash[b]{\block{k}} \\
  && 1 & \smash[b]{\block{k}} \\
  &&&& \ddots \\
  &&&&& 1 & \block{k}
  \end{pmatrix}
}_{T}
$
\end{document}

enter image description here

If you don't want to space the dots in the underbraced blocks, then change the definition:

\documentclass{standalone}
\usepackage{amsmath}

\newcommand{\block}[1]{
  \underbrace{1 \cdots 1}_{#1}
}

\begin{document}
$
\underbrace{
  \begin{pmatrix}
  1 & \smash[b]{\block{k}} \\
  && 1 & \smash[b]{\block{k}} \\
  &&&& \ddots \\
  &&&&& 1 & \block{k}
  \end{pmatrix}
}_{T}
$
\end{document}

enter image description here

For getting the underbrace only inside the delimiters, it's more complicated, because we don't want that the underbrace is considered when sizing the delimiters, yet we want to consider the vertical space taken by it.

\documentclass{standalone}
\usepackage{amsmath}

\newcommand{\block}[1]{
  \underbrace{1 \cdots 1}_{#1}
}

\newcommand{\underbracedmatrix}[2]{%
  \left(\;
  \smash[b]{\underbrace{
    \begin{matrix}#1\end{matrix}
  }_{#2}}
  \;\right)
  \vphantom{\underbrace{\begin{matrix}#1\end{matrix}}_{#2}}
}

\begin{document}
$
\underbracedmatrix{
  1 & \smash[b]{\block{k}} \\
  && 1 & \smash[b]{\block{k}} \\
  &&&& \ddots \\
  &&&&& 1 & \block{k}
}{T}
$
\end{document}

enter image description here

edited Feb 21 '17 at 21:08

barbara beeton

88,848

answered Nov 19 '13 at 00:50

egreg

1,121,712

1

If I interprete the OP correct, the brace should not include the surrounding Parentheses. – Svend Tveskæg Nov 19 '13 at 20:26
@SvendTveskæg I added the version you asked for. – egreg Nov 19 '13 at 22:16
Nice. I think the OP---not me---asked for it. :) – Svend Tveskæg Nov 20 '13 at 04:41
@egreg You've put a lot in to this answer. Nice work. – physicsmichael Jan 16 '14 at 19:03
What does the [b] option do to \smash? – Daniel Kessler Feb 24 '21 at 01:42
1

@DanielKessler It just removes the depth of the box, so that TeX doesn’t think that the box extends below the baseline. In the particular case it’s not really needed, because every row has a 1 that’s not smashed., but it would be needed otherwise. – egreg Feb 24 '21 at 08:02

score 3 · Answer 2 · edited Jun 09 '17 at 00:02

3

Just another flavor even though @egreg solutions seems to be robust enough:

\documentclass{standalone}

\usepackage{amsmath,mathtools}
\newcommand{\sunderb}[2]{
  \mathclap{\underbrace{\makebox[#1]{$\cdots$}}_{#2}}
}
\begin{document}
$
\begin{pmatrix}
1 & 1 & 1 & 1\\
1 & 1 & \sunderb{3.5em}{k} & 1 \\ 
1 & 1 & 1 & 1
\end{pmatrix}
$
\end{document}

enter image description here

edited Jun 09 '17 at 00:02

David Carlisle

757,742

answered Nov 19 '13 at 00:57

azetina

28,884

score 3 · Answer 3 · edited Feb 21 '17 at 21:09

3

Yet another attempt:

\documentclass{article}

\begin{document}

\def\onegroup{1\hskip\arraycolsep\underbrace{1 \hskip\arraycolsep\dots \hskip\arraycolsep 1}_{k}}

\[
\underbrace{
\left(
\begin{array}{cccc}
\onegroup&&&\\
&\onegroup&&\\
&&\ddots&\\
&&&\onegroup
\end{array}
\right)
}_{T}
\]

\end{document}

enter image description here

edited Feb 21 '17 at 21:09

barbara beeton

88,848

answered Nov 19 '13 at 00:59

Przemysław Scherwentke

37,268

The output of the code does not match the image which you have pasted, in particular it is missing the large underbrace with the T at the bottom. – I Like to Code Nov 19 '13 at 15:01
@ILiketoCode Oh! It was an earlier version of code. Corrected. – Przemysław Scherwentke Nov 19 '13 at 20:17

Underbrace in a matrix

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