[Tex/LaTex] Underbrace in a matrix

amsmathmath-modematrices

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}

Best Answer

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

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