parenthesis around underbrace too big

Question

I have the problem that I want to have an equation with underbrace but the surrounding parenthesis shall not include the underbrace, i.e. should have the size as if the underbrace wasn't there. Here is the code I use:

\documentclass{article}
\usepackage{amsmath}
\begin{document}    
$\exp \left(\gamma \underbrace{\begin{pmatrix} 1 & 2\\ 3 & 4 \end{pmatrix}}_\text{some text} \right)$
\end{document}

I'd like to have that the outer parenthesis have the same size and center as the parenthesis of the pmatrix. I tried \Bigg( but it looked slightly off and it only works for 2x2 matrices but not for 3x3. Any ideas?

Answer 1

this is overkill for a 2-by-2 matrix, but it would work for larger matrices as well, even for ones where big versions of parentheses aren't available.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$\exp \left( {\vphantom{\begin{pmatrix} 1\\ 3 \end{pmatrix}}} \right. \!\!
\gamma \underbrace{\!\! \begin{pmatrix} 1 & 2\\
 3 & 4 \end{pmatrix} \!\!}_\text{some text} \!\!
\left. {\vphantom{\begin{pmatrix} 1\\ 3 \end{pmatrix}}} \right)$
\end{document}

observe the use of \! to get rid of some unwanted horizontal space.

Answer 2

Maybe I shouldn't have posted this but I'm reviewing a lot of papers involving huge matrices and I can't stand inline matrices. If needed use the smallmatrix environment or state the matrix exponential as $e^{\gamma A}$ and put A matrix as a display math.

\documentclass{article}
\usepackage{mathtools} %<- Builds on top of amsmath and fixes/improves some stuff
\begin{document}    
The sentences start like this then slowly they get longer and longer and then sentence
becomes like this 
$e^{\gamma\!\! 
\underbracket[0.4pt][1pt]{
\begin{psmallmatrix} 1 & 2\\ 3 & 4 \end{psmallmatrix}
}_{\text{\tiny some text}} }$ 
and maybe after that something like 
\raisebox{0.5ex}{%
$\exp{(\gamma\!\! \underbracket[0.4pt][1pt]{
\begin{psmallmatrix} 1 & 2\\ 3 & 4 \end{psmallmatrix}}_{\text{\tiny some text}} )}
$
}
then continues with more stuff that starts another line which still looks terrible.
However this one 
\[
e^{\left(\gamma\!\! \underbracket[0.4pt][1pt]{
\begin{psmallmatrix} 1 & 2\\ 3 & 4 \end{psmallmatrix}
}_{\text{\tiny some text}} \right)} = 
\exp{(\gamma\!\! \underbracket[0.4pt][1pt]{
\begin{psmallmatrix} 1 & 2\\ 3 & 4 \end{psmallmatrix}
}_{\text{\tiny some text}} )}
\]
has no such problems. But still none of them works properly
in my opinion. The best is to go with $e^{\gamma A}$ where 
\[
A =\begin{pmatrix}
     1&2\\3&4
\end{pmatrix}
\]
\end{document}

Answer 3

If you need it only once, I would go for a manual adjustment:

\documentclass{article}
\usepackage{amsmath}
\begin{document}    
$\exp
\mkern30mu
\underbrace{
\mkern-30mu
\left(
\gamma
\begin{pmatrix} 1 & 2\\ 3 & 4 \end{pmatrix}
\right)
\mkern-18mu
}_\text{some text}
\mkern18mu
$
\end{document}

I put the \underbrace outside, and using \mkern with positive and negative value I made it think that its contents is smaller. The values 30mu and 18mu were found by trial-and-error.

Answer 4

Here is another:

\documentclass{article}
\usepackage{amsmath}
\newsavebox{\bpmatrixbox}
\newdimen\parenwd \parenwd=8.75002pt
\newcommand\bpmatrix[2]{
  \savebox{\bpmatrixbox}{\(\begin{pmatrix}#1\end{pmatrix}\)}
  \vrule height\ht\bpmatrixbox depth\dp\bpmatrixbox width0pt
  \smash{\oalign{
    \usebox{\bpmatrixbox}\cr\noalign{\kern\jot}
    \kern\parenwd\upbracefill\kern\parenwd\cr
    \strut\hidewidth\small #2\hidewidth}}
}

\begin{document}
Some text before
\begin{gather*}
  \exp\left(\gamma \bpmatrix{1& 2\\ 3& 4}{some text} \right) \\
\end{gather*}
the previous display math environment clashes into this here text without the gather's last double backslashes.
\end{document}

Answer 5

Again, overkill; but, for what it's worth, here's a tikz solution that appears to work.

The primary difficulty here, as elsewhere, was getting the underbrace and its text to count for vertical spacing, but not until after the outer parentheses were set. The tikz solution I initially had in mind was to save the position with tikz nodes in the middle of the math, and then come back afterwards with an underbrace at the saved position. The trouble with this approach is that tikz constructions that reference positions from other tikz pictures are supposed to be given the overlay option, which effectively smashes any effect the construction would have on the vertical spacing (and would make the next line too high).

The way I got around this was to put the entire expression in a node in a tikz picture. Thus, I actually have tikz nodes inside a tikz picture inside a tikz node inside a tikz picture. I'm not sure whether this behavior is supported, but it seems more or less to work for this particular construction.

\documentclass{article}
\usepackage{amsmath,tikz}
\usetikzlibrary{decorations.pathreplacing}
\usetikzlibrary{calc}
\newcommand{\currentmatrix}{}
\newcommand{\tikznode}[2][$\currentmatrix$]{\tikz[overlay]\node (#2) {\vphantom{#1}};}

\begin{document}
\renewcommand{\currentmatrix}{\begin{pmatrix} 1 & 2\\ 3 & 4 \end{pmatrix}}
\begin{align*}
\tikz[remember picture,baseline=(left.east),inner xsep=0pt]{
    \node {$\exp \left(\gamma \tikznode{left}
        \begin{pmatrix} 1 & 2\\ 3 & 4 \end{pmatrix}\tikznode{right} 
        \right)$};
    \draw[overlay,decorate,decoration={brace,amplitude=3pt}]
        ($(right.south west) + (0,4pt)$) -- 
        coordinate(texthere) ($(left.south east)+(0,4pt)$); 
    \path (texthere) node[below] {$\scriptstyle\text{some text}$};
} &= e^{\gamma A} \\
&= \exp(\gamma A)
\end{align*}

\end{document}