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I want to place text under an entire multivariate expression, exactly the way one is able with \underbrace, but without the brace. I want it to specifically describe the first variable in the expression, but for the text to span the entire expression. I have scoured the webs and it's all about placing text under single symbols or commands. For example, \underset, \smashoperator, and \limits can't seem to function with more than one command or variable, even when I use brackets around the command line.

I got what I wanted using this:

    \begin{align*}
      N_{e}(r)\, =\, \sum_{u, v, w}\; &n_{e}\,(\overrightharp{\text{r}}\, -\,  u\overrightharp{\text{a}}\, -\, v\overrightharp{\text{b}}\, -\,  w\overrightharp{\text{c}}) \\[-.4cm]
       & \lcurvearrownw \text{\footnotesize{electron density per}}\\[-.2cm]
       &\; \; \; \; \; \; \; \text{\footnotesize{primitive unit cell.}}
    \end{align*}

but I was wondering if there was a less hacky way of going about it. I want my client to be able to feasibly edit their documents when I am finished. In other words, I want them to know what is going on here.

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Welcome to TeX.SE. It is always best to compose a fully compilable MWE that illustrates the problem including the \documentclass and the appropriate packages so that those trying to help don't have to recreate it. –  Peter Grill Sep 3 '12 at 5:25
    
One way to do this would be to use \tikzmark: You could use something like Asymmetric overbrace and use draw=none so that that brace is not drawn, or if you want something more elaborate Curly brace to insert something into an equation? Like an inverted underbrace. –  Peter Grill Sep 3 '12 at 5:27
    
Thank you Peter. –  Brazos Wolf Sep 3 '12 at 5:28

1 Answer 1

up vote 7 down vote accepted

You can use a \parbox to write the text; using \mathrclap (from the mathtools package) and \raisebox you can place the \parbox in the desired position; here's a simple example using a defined command \Desc with two mandatory arguments (the box width and the text) and one optional argument controlling the vertical shifting:

\documentclass{article}
\usepackage{mathtools,MnSymbol,harpoon}

\newcommand\Harp[1]{%
  \overrightharp{\textrm{#1}}}
\newcommand\Desc[3][-2.5ex]{%
  \mathrlap{\raisebox{#1}{$\lcurvearrownw$ \parbox[t]{#2}{\footnotesize #3}}}}

\begin{document}

\[
N_{e}(r) = \sum_{\mathclap{u, v, w}}\, n\Desc{3cm}{electron density per \\ primitive unit cell.}_{e}(\Harp{r} - u\Harp{a} - v\Harp{b} - w\Harp{c}) 
\]

\end{document}

enter image description here

Since you are using the command \lcurvearrownw I assumed that you were loading the MnSymbol package; this package changes many symbols.

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1  
The usual caveat that MnSymbol changes many symbols should appear. –  egreg Sep 3 '12 at 10:23
    
@egreg remark added. Thanks! –  Gonzalo Medina Sep 3 '12 at 13:16
    
Thank you @ Gonzalo, that is much simpler than my original solution and I've noted MnSymbol's potential problems. –  Brazos Wolf Sep 8 '12 at 20:04

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