[Tex/LaTex] How to write above a left-right arrow

arrowsmath-mode

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.

Best Answer

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}

enter image description here

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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