Underbrace in Matrix without array package

Question

I've been struggling with matrix underbrace, and I can't find any way through all LaTeX questions about how to use an underbrace in a matrix without the array package.

So far I got here, with these new commands, but the underbrace/overbrace is not correctly placed in the second matrix.

MWE

\documentclass[12pt]{report}
\usepackage{amsmath, amssymb, etoolbox}
\newcommand\aug{\fboxsep=-\fboxrule!!!\fbox{\strut}!!!}
\newcommand\undermat[2]
    {\makebox[0pt][l]{$\smash{\underbrace{\phantom{\begin{matrix}#2\end{matrix}}}_{\text{$#1$}}}$}#2}
\newcommand\overmat[2]{
  \makebox[0pt][l]{$\smash{\overbrace{\phantom{\begin{matrix}#2\end{matrix}}}^{\text{$#1$}}}$}#2}
\begin{document}
$$\begin{pmatrix}
    2 & 3 & 1 & \aug & 1 & 0 & 0 \
    1 & 0 & 1 & \aug & 0 & 1 & 0 \
   \undermat{A}{ 5 & 4 & 6 } & \aug & \undermat{I}{ 0 & 0 & 0 }\
\end{pmatrix}
\xrightarrow[\rm{Jordan}]{\rm{Gauss}}
\begin{pmatrix}
    \overmat{I}{ 1 & 0 & 0 }& \aug & \overmat{A^{-1}}{ 4/7 & 2 & -3/7 }\
    0 & 1 & 0 & \aug & 1/7 & -1 & 1/7 \
    0 & 0 & 1 & \aug & -4/7 & -1 & 3/7 \
\end{pmatrix}$$
\end{document}

In the second matrix, if I use \undermat{}{} it gives the same result.

why not array package (it is part of core latex, unlike say etoolbox which you are using) (I don't think it would help much here, it just seems a strange requirement in your question. The only reason array is not pre-loaded in the format is memory requirements on PCs in 1993 — David Carlisle, Nov 04 '23 at 12:28
\rm is not defined by default in latex, but where it is defined for compatibility with pre-1993 documents, the syntax is {\rm Jordan}] not \rm{Jordan} it did not take an argument. — David Carlisle, Nov 04 '23 at 12:30
I forgot, i renamed the command \textrm to \rm to make it faster. I don't like the array package because I got a lot of trouble with incompatibilites with the xltabular package and a bunch others. — Agente 156, Nov 04 '23 at 12:32
that's a pretty bad choice, \rm has been known to tex syntax checkers editor highlighting etc for nearly 40 years defining a new command of the same name but different syntax will confuse everything, not just me. — David Carlisle, Nov 04 '23 at 12:35
Your restriction has moved from strange, to simply wrong. xltabular uses tabularx which is based on array by the same author, me:-) so you are loading array. — David Carlisle, Nov 04 '23 at 12:37
Ok, sorry, i didn't test enough the array package and I was skeptical about it and the incompatibilities, I'm probably going to delete this question, but now that you are here, the creator of xltabular, can I ask you how to move the title of a section to a new page if an xltabular environment inmediatly breaks page. I mean that there is some cases were the title of the section is in the last line of the page and the xltabular starts in a new page and it looks weird. Thank you for you LaTeX colaborations — Agente 156, Nov 04 '23 at 12:52
it shouldn't happen, but if it does, simplest is probably to just put \clearpage before the \section — David Carlisle, Nov 04 '23 at 13:21

score 0 · Answer 1 · answered Nov 04 '23 at 14:04

This might be better syntax-wise, but I don't think you have too many of these constructions in your document. Only normal sized cells are supported.

\documentclass[12pt]{report}
\usepackage{amsmath}
\usepackage{lipsum}
\NewDocumentCommand{\undermat}{mm}{%
  \vphantom{\begin{matrix}#2\end{matrix}}%
  \smash{,\underbrace{\begin{matrix}#2\end{matrix}}_{#1},}%
}
\NewDocumentCommand{\overmat}{mm}{%
  \vphantom{\begin{matrix}#2\end{matrix}}%
  \smash{,\overbrace{\begin{matrix}#2\end{matrix}}^{#1},}%
}
\ExplSyntaxOn
\cs_set_eq:NN \Replicate \prg_replicate:nn
\ExplSyntaxOff
\NewDocumentCommand{\fixunder}{m}{%
  \vphantom{%
    \underbrace{\begin{matrix}\Replicate{#1-1}{\}\end{matrix}}_{\mathstrut}%
  }%
}
\NewDocumentCommand{\fixover}{m}{%
  \vphantom{%
    \overbrace{\begin{matrix}\Replicate{#1-1}{\}\end{matrix}}^{\mathstrut}%
  }%
}
\begin{document}
\lipsum[1][1-4]
[
\fixunder{3}\fixover{3}
\left(
  \undermat{A}{
    2 & 3 & 1 \
    1 & 0 & 1 \
    5 & 4 & 6
  }
;\middle|;
  \undermat{I}{
    1 & 0 & 0 \
    0 & 1 & 0 \
    0 & 0 & 0
   }
\right)
\xrightarrow[\textrm{Jordan}]{\textrm{Gauss}}
\left(
  \overmat{I}{
    1 & 0 & 0 \
    0 & 1 & 0 \
    0 & 0 & 1
  }
;\middle|;
  \overmat{A^{-1}}{
    4/7 &  2 & -3/7 \
    1/7 & -1 &  1/7 \
   -4/7 & -1 &  3/7
  }
\right)
]
\lipsum[1][1-4]
\end{document}

Note the \fixover and \fixunder commands that take as argument the number of rows that should be considered.

Avoid $$ in LaTeX, see Why is \[ ... \] preferable to $$ ... $$?. In the sense that you should never use it inside document (there are a few cases in which $$...$$ is useful in defining commands or environments).

In this particular case, when the line before the display is short, you migh avoid \fixover{3} to get

Underbrace in Matrix without array package

MWE

1 Answers1