A way to mark things in math mode

Question

I want to mark the weight function in an equation, using something similar to \underbrace. Using this piece of code:

(x^2y')' + \lambda \underbrace{x^2}_{\text{Weight function}}y = 0

I get

.

I don't want such a big space between the x² and the other parts of the equation. I wouldn't mind, for example, having an arrow leading downwards, and there to write the description. Is there any way to do what I describe?

Answer 1

Here I've played around with different ways of highlighting x² without resorting to any (major) additional packages. There are ways of doing it more elegantly (using tikz, for example):

\documentclass{article}
\usepackage{grahpicx}% http://ctan.org/pkg/graphicx
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\[
  (x^2y')' + \lambda \underbrace{x^2}_{\text{\makebox[0pt]{Weight function}}}y = 0
\]

\[
  (x^2y')' + \lambda \mathop{\rlap{\resizebox{1em}{\ht\strutbox}{$\underbrace{\phantom{x^2}}$}}x^2}_{\text{\makebox[0pt]{Weight function}}}y = 0
\]

\[
  (x^2y')' + \lambda
  \begin{array}[t]{@{\,}c@{\,}}
    x^2 \\ \downarrow \\ \makebox[0pt]{\scriptsize Weight function}
  \end{array}
  y = 0 
\]

\[
  (x^2y')' + \lambda
  \begin{array}[t]{@{}c@{}}
    \setlength{\fboxsep}{1pt}
    \fbox{$x^2$} \\ \downarrow \\ \makebox[0pt]{\scriptsize Weight function}
  \end{array}
  y = 0 
\]

\[
  (x^2y')' + \lambda \underset{\text{\makebox[0pt]{Weight function}}}{\underset{\downarrow}{x^2}}  y = 0 
\]
\end{document}

Option 1 is fairly basic. Option 2 sets a fake x² with a appropriate \underbrace, but resizes it to the width of 1em, before typesetting the rest of the expression as \mathop. Options 3 & 4 use an array to stack symbols. Option 5 uses a stacked-\underset.

Answer 2

The mathtools package gives the command \mathclap, which can be used to give

\documentclass{article}

\usepackage{mathtools}

\begin{document}
\[
    (x^2y')' + \lambda \underbrace{x^2}_{\mathclap{\text{Weight function}}}y = 0
\]
\end{document}

Answer 3

Here's a simple way:

\let\hw\hidewidth
$$
  (x^2y')'+\lambda \underbrace{\mathop{x^2}}_{\hw\fam0 Weight~function\hw} y = 0
$$
\bye