Shrinking of union or intersection symbol for the intervals

Question

We use this very short MWE as an example. It surprises me that when constructing intervals with union or intersection symbols, LaTeX does not provide a minimum of space between a square bracket and these symbols. Visibly it all looks compressed.

What is the reason and what could be a solution without inserted spaces (or to use the spaces)?

\documentclass[12pt]{article}

\usepackage[margin=2cm]{geometry}
\usepackage[utf8]{inputenc} 
\usepackage[T1]{fontenc}

\usepackage{mathtools,amssymb}
\begin{document} 
$]-\infty,-1[\cup]-1,1[\cup]1,+\infty[$.
\end{document}

Answer 1

You need to tell TeX you are using inverted brackets for intervals

\documentclass[12pt]{article}

\usepackage[margin=2cm]{geometry}
\usepackage[utf8]{inputenc} 
\usepackage[T1]{fontenc}

\usepackage{mathtools,amssymb}
\begin{document} 
$\mathopen]-\infty,-1\mathclose[
\cup
\mathopen]-1,1\mathclose[
\cup
\mathopen]1,+\infty\mathclose[$.
\end{document}