How to create this arrow with letters above and below?

Question

How to create this arrow with letters above and below?

Answer 1

The mathtools package offers a wide variety of arrows, which can have content above and below.

\documentclass{standalone}   
\usepackage{mathtools}
\begin{document} 
$\xleftrightarrow[\mathcal{F}^{-1}]{\mathcal{F}}$
\end{document}

Answer 2

You can try:

\documentclass{report}
\usepackage{mathtools}

\begin{document}

\[
\xleftrightarrow[F^{-1}]{F}
\]
\end{document}

See http://en.wikibooks.org/wiki/LaTeX/Advanced_Mathematics#Above_and_below or mathtools-documentation for more variants.