documentclass{minimal}
usepackage{mathtools}
DeclarePairedDelimiterleftopenint{(}{]}
DeclarePairedDelimiterrghtopenint{[}{)}

begin{document}
  begin{equation*}
   leftopenint{-infty, frac{1}{N} } cup rghtopenint*{frac{1}{N}, infty}
  end{equation*}
end{document}