How avoid that the Underbrace Outside the delimiters $| |$ appear inside the delimiters

Question

I have this code.

\documentclass[12pt,a4paper]{article}

\usepackage[utf8]{inputenc}
\usepackage[spanish]{babel}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{pgf,tikz}
\usetikzlibrary{arrows}
\usepackage{float}
\usepackage{mathtools}
\begin{document}

\[
\left|\underbrace{-6rO^2\left(\frac{ G_p}{r}\right)-3O^2\left(\frac{ G_p}{r}\right)}_{O^1\left(\frac{ G_p}{r}\right)}\right|
\end{document}
\]

Im trying to make that the underbrace appear outside the delimiter $||$ but if I colocate the underbrace outside the delimiter, like this

\[
\underbrace{\left|-6rO^2\left(\frac{ G_p}{r}\right)-3O^2\left(\frac{ G_p}{r}\right)}_{O^1\left(\frac{ G_p}{r}\right)\right|}
\]

I get an error. How to solve it?

Answer 1

You have to pay attention to proper nesting of braces and \left ... \right pairs, your second example is incorrect in this sense. Just avoid using \left ... \right since they are evil, and you'll be fine:

\documentclass[12pt,a4paper]{article}

\usepackage[utf8]{inputenc}
\usepackage[spanish]{babel}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{pgf,tikz}
\usetikzlibrary{arrows}
\usepackage{float}
\usepackage{mathtools}
\begin{document}

\[
\underbrace{\biggl|-6rO^2\Bigl(\frac{ G_p}{r}\Bigr)-3O^2\Bigl(\frac{G_p}{r}\Bigr)\biggr|}_{O^1\bigl(\frac{ G_p}{r}\bigr)}
\]

\end{document}

Btw, if I just change \left ... \right to either \biggl ... \biggr or \Bigl ... \Bigr or \bigl ... \bigr depending on how large delimiters I want, it's immediately obvious where is the mistake:

\documentclass[12pt,a4paper]{article}

\usepackage[utf8]{inputenc}
\usepackage[spanish]{babel}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{pgf,tikz}
\usetikzlibrary{arrows}
\usepackage{float}
\usepackage{mathtools}
\begin{document}

\[
\underbrace{\biggl|-6rO^2\Bigl(\frac{ G_p}{r}\Bigr)-3O^2\Bigl(\frac{G_p}{r}\Bigr)}_{O^1\bigl(\frac{ G_p}{r}\bigr)\biggr|}
\]

\end{document}

Answer 2

If you need flexibility in how the brace is drawn then you could use tikz as well:

Code:

\documentclass[12pt,a4paper]{article}

\usepackage[utf8]{inputenc}
\usepackage{pgf,tikz}
\usetikzlibrary{arrows,calc,decorations.pathreplacing}
\usepackage{mathtools}
\usepackage{xparse}

\newcommand{\tikzmark}[1]{\tikz[overlay,remember picture] \node (#1) {};}

% Tweak these as necessary
\newcommand*{\BraceAmplitude}{0.4em}%
\newcommand*{\VerticalOffset}{1.0ex}%  
\newcommand*{\HorizontalOffset}{0.0em}% 


\NewDocumentCommand{\InsertUnderBrace}{%
    O{} % #1 = draw options
    O{yshift=0.0cm} % #2 = optional brace shift options
    m   % #3 = left tikzmark
    m   % #4 = right tikzmark
    m   % #5 = text to place under brace
}{%
    \begin{tikzpicture}[overlay,remember picture]
    \draw [decoration={brace, amplitude=\BraceAmplitude}, decorate, thick, draw=blue, text=black, #1]
        ([#2]$(#4)+( \HorizontalOffset,-\VerticalOffset)$) -- 
        ([#2]$(#3)+(-\HorizontalOffset,-\VerticalOffset)$)
        node [below=\VerticalOffset, midway, align=center] {#5};
    \end{tikzpicture}%
}%

\NewDocumentCommand{\AbsWithUnderBrace}{%
    s%               #* = draw brace on outside
    O{}%             #2 = draw options
    O{yshift=0.0cm}% #3 = optional brace shift options
    m%               #4 = content
    m%               #4 = underbrace content
}{%
    \IfBooleanTF{#1}{%
        \tikzmark{StartBraceA}\left|#4\right|\tikzmark{EndBraceA}
    }{%
        \left|\tikzmark{StartBraceA}#4\tikzmark{EndBraceA}\right|
    }%
    \InsertUnderBrace[draw=red, ultra thick, text=blue][yshift=-1.5ex]{StartBraceA}{EndBraceA}
        {$O^1\left(\frac{ G_p}{r}\right)$}%
}

\begin{document}
\[
    \AbsWithUnderBrace*[draw=red, ultra thick, text=blue][yshift=-1.5ex]
        {-6rO^2\left(\frac{ G_p}{r}\right)-3O^2\left(\frac{ G_p}{r}\right)}
        {$O^1\left(\frac{ G_p}{r}\right)$}
\qquad
    \AbsWithUnderBrace[draw=red, ultra thick, text=blue][yshift=-1.5ex]
        {-6rO^2\left(\frac{ G_p}{r}\right)-3O^2\left(\frac{ G_p}{r}\right)}
        {$O^1\left(\frac{ G_p}{r}\right)$}
\]
\end{document}

Answer 3

A few possible improvements to previous (Tohecz's) solution: since you load mathtools (in that case, needless to load amsmath), you can define an \abs command that can have either an optional argument: \big, \Big and the like, or a star version that implicitly adds a pair of \left…\right. This will result in a cleaner syntax.

You also can use the \underbracket command, with an adjustable thickness.

Demo:

    \documentclass[12pt,a4paper]{article}

    \usepackage[utf8]{inputenc}
    \usepackage[spanish]{babel}
    \usepackage{amsfonts}
    \usepackage{amssymb}
    \usepackage{mathtools}
    \usepackage{etoolbox}


    \DeclarePairedDelimiterX{\abs}[1]{\lvert}{\rvert}{\ifblank{#1}{\: · \:}{#1}}
    \begin{document}

    \[
    \underbrace{\abs[\Big]{-6rO^2\Bigl(\frac{ G_p}{r}\Bigr)-3O^2\Bigl(\frac{G_p}{r}\Bigr)}}_{O^1\bigl(\frac{ G_p}{r}\bigr)}
    \qquad
    \underbracket[0.6pt]{\abs[\Big]{-6rO^2\Bigl(\frac{ G_p}{r}\Bigr)-3O^2\Bigl(\frac{G_p}{r}\Bigr)}}_{O^1\bigl(\frac{ G_p}{r}\bigr)}
    \]
    \[
    \underbrace{\abs*{-6rO^2\Bigl(\frac{ G_p}{r}\Bigr)-3O^2\Bigl(\frac{G_p}{r}\Bigr)}}_{O^1\bigl(\frac{ G_p}{r}\bigr)}
    \qquad
    \underbracket[0.6pt]{\abs*{-6rO^2\Bigl(\frac{ G_p}{r}\Bigr)-3O^2\Bigl(\frac{G_p}{r}\Bigr)}}_{O^1\bigl(\frac{ G_p}{r}\bigr)}
    \]
    \end{document} 

Answer 4

You want to typeset only the main part without the subscript and the underbrace to get the required height of the delimiters, which can be done with \vphantom.

The syntax of \dunderbrace is

\dunderbrace<left><body>_<subscript><right>

where <left> is the left delimiter (in your case |), <body> is to be underbraced, <subscript> is what goes under the brace and <right> is the right delimiter.

Note that _<subscript> can be omitted.

\documentclass[a4paper]{article}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mleftright}
\mleftright

\usepackage{lipsum} % just for the example

\makeatletter
\newcommand{\dunderbrace}[2]{%
  \@ifnextchar_{\dunder@next{#1}{#2}}{\dunder@next{#1}{#2}_{}}%
}
\def\dunder@next#1#2_#3#4{%
  \left#1\vphantom{\displaystyle#2}\right.\n@space
  \underbrace{#2}_{#3}%
  \n@space\left.\vphantom{#2}\right#4%
}
\makeatother

\begin{document}

\lipsum*[2]
\[
\dunderbrace|{-6rO^2\left(\frac{G_p}{r}\right)-3O^2\left(\frac{G_p}{r}\right)}
  _{O^1\left(\frac{G_p}{r}\right)}|
\]
\lipsum[3]

\end{document}

If your aim is that the underbrace also cover the delimiters, then it's much easier:

\[
\underbrace{\left|-6rO^2\left(\frac{G_p}{r}\right)-3O^2\left(\frac{G_p}{r}\right)\right|}
 _{O^1\left(\frac{G_p}{r}\right)}
\]