French notation for intervals ]x,y[
you can use \mathopen]x,y\mathclose[
The interval
package by Lars Madsen has been designed to take care of this kind of problem (and some others):
\documentclass{article}
\usepackage{interval}
\begin{document}
\[
\interval[open]{x}{y} \cup \interval[open]{a}{b}
\]
\end{document}
See its documentation for more information.
If you have many of these intervals, you may want to create a dedicated macro called, say, \frint
(short for "French-notation interval"...), as follows:
\documentclass{article}
\usepackage{mathtools} % for "\DeclarePairedDelimiter" macro
\DeclarePairedDelimiter{\frint}{]}{[}
\begin{document}
$\frint{x,y}\cup\frint{a,b}$
%% same output as '$\mathopen{]}x,y\mathclose{[} \cup \mathopen{]}a,b\mathclose{[}$'
\end{document}
Then, write \frint*
to auto-size the "fences", and write \frint[\big]
, \frint[\Big]
, \frint[\bigg]
, or \frint[\Bigg]
to enlarge the fences to a specific, fixed size.