I would like to have text (maybe a bullet, maybe something else) above a left-right arrow, I would also like that text to be centred. I would also like the arrow to grow with the text.
MWE:
\documentclass{article}
\usepackage{mathtools}
\begin{document}
$a \xleftrightarrow{x} b$
\end{document}
I tried this answer but it produces results similar to the above.
The definition of \xleftrightarrow is wrong.
\documentclass{article}
\usepackage{mathtools}
\MHInternalSyntaxOn\makeatletter
\renewcommand{\xleftrightarrow}[2][]{\ext@arrow 5599\MT_leftrightarrow_fill: {#1}{#2}}
\makeatother\MHInternalSyntaxOff
\newcommand{\bb}[1]{\mathpalette\bbx{#1}}
\newcommand{\bbx}[2]{{\fboxsep=-0.05pt\fboxrule=0.1pt\fbox{$#1#2$}}}
\begin{document}
$a \xleftrightarrow{\bb{x}} b$
\end{document}
The macro \bb is just used for showing the bounding box of the label, you don't need it.
mathtools?! – Sigur Apr 14 '20 at 11:55