overbrace/underbrace but with an arrow instead

Question

Is there a similar command to \overbrace{foo}^{bar} but instead of braces, an arrow pointing up/down?

EDIT: Here's a little demonstration of how I'd expect something like this to work:

And possible syntax would be \overarrow{a=b}^{\text{theorem X}}

Answer 1

Here is a generalized approach, providing \overarrow[<relation>][<arrow>]{<stuff>}. By default <relation> is = and <arrow> is \uparrow (both arguments are optional). A complement to this is provided by \underarrow[<relation>][<arrow>]{<stuff>} for typesetting underneath <operator> (<arrow> has the default <downarrow> in \underarrow).

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\usepackage{xparse}% http://ctan.org/pkg/xparse

\NewDocumentCommand{\overarrow}{O{=} O{\uparrow} m}{%
  \overset{\makebox[0pt]{\begin{tabular}{@{}c@{}}#3\\[0pt]\ensuremath{#2}\end{tabular}}}{#1}
}
\NewDocumentCommand{\underarrow}{O{=} O{\downarrow} m}{%
  \underset{\makebox[0pt]{\begin{tabular}{@{}c@{}}\ensuremath{#2}\\[0pt]#3\end{tabular}}}{#1}
}

\begin{document}
\begin{align*}
  \alpha &\overarrow{Theorem X}\beta & \alpha &\underarrow{Theorem X}\beta  \\
  \alpha &\overarrow[\approx]{Theorem Y}\beta & \alpha &\underarrow[\approx]{Theorem Y}\beta \\
  \alpha &\overarrow[>][\downarrow]{Theorem Z}\beta & \alpha &\underarrow[>][\uparrow]{Theorem Z}\beta
\end{align*}

\end{document}

One difference between this typesetting and that proposed by @Herbert is that \stackrel modifies the size of the upper component. Since \overarrow and \underarrow in this answer uses tabular to stack the items, no size adjustment is visible.

xparse provides the interface for mixing optional/mandatory arguments.

Answer 2

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\def\overUnderArrow{\@ifnextchar[\overUnderArrow@i{\overUnderArrow@i[]}}
\def\overUnderArrow@i[#1]#2#3{% #1 under #2 over #3 main argument
  \ifx\relax#1\relax\array[b]{c}\overset{\text{#2}}{\uparrow}\\#3\endarray
  \else\ifx\relax#2\relax
    \array[t]{c}#3\\\underset{\text{#1}}{\downarrow}\endarray
  \else
    \array{c}\overset{\text{#2}}{\uparrow}\\#3\\\underset{\text{#1}}{\downarrow}\endarray
  \fi\fi}
\makeatother
\begin{document}
\[
  \overUnderArrow[TheoremX]{TheoremX}{\alpha = \beta}
  \overUnderArrow{TheoremX}{\alpha = \beta}
  \overUnderArrow[TheoremX]{}{\alpha = \beta}
\]

\end{document}

The syntax is \overUnderArrow[below]{above}{main entry}. It is also possible to extend the definition for variable arrows direction.