# How do I write above a left-right arrow?

There are a few articles on how to do this for left arrow or right arrow but the method doesn't work for \leftrightarrow.

What I want is to write a \longleftrightarrow with “DFT” above it.

I've tried:

\longleftrightarrow{\text{DFT}}
\longleftrightarrow^{DFT}
\xlongleftrightarrow{\text{DFT}}
\longleftrightarrow\limits^{DFT}


None has worked.

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You can define an extensible "leftright arrow" by the method explained in Section 38 in Mathmode.pdf by Herber Voß. You can read it with texdoc mathmode on your machine or via this link.

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcommand\xleftrightarrow[2][]{%
\ext@arrow 9999{\longleftrightarrowfill@}{#1}{#2}}
\newcommand\longleftrightarrowfill@{%
\arrowfill@\leftarrow\relbar\rightarrow}
\makeatother

\begin{document}

$F \xleftrightarrow{\text{DFT}} G$

$F \xleftrightarrow{\text{overlong text}} G$

\end{document}


The optional argument to \xleftrightarrow is for a "under label", so \xleftrightarrow[xyz]{} will have xyz under the arrow, while \xleftrightarrow[xyz]{abc} will have abc over and xyz under the arrow.

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They are already provided by mathtools ;-) –  daleif Feb 4 '13 at 9:21
\stackrel{\mathrm{DFT}}{\longleftrightarrow}

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