saito.tex

\documentclass{article}
\usepackage{amsmath}
\usepackage[active,tightpage]{preview}
\setlength\PreviewBorder{2mm}

\makeatletter
\newcommand{\oopen}{%
  \bgroup\hskip-.4em
    \mathop{(}\limits^{\vbox to -1.2\ex@{\kern-\tw@\ex@
    \hbox{\hskip.4em\scriptsize\ensuremath{\circ}}\vss}}%
  \egroup}
\newcommand{\oclose}{%
  \bgroup\hskip.3em
    \mathop{)}\limits^{\vbox to -1.2\ex@{\kern-\tw@\ex@
    \hbox{\hskip-.4em\scriptsize\ensuremath{\circ}}\vss}}%
  \egroup}
\makeatother

\begin{document}
\begin{preview}
$\oopen a, b\oclose$
\end{preview}
\end{document}

斎藤正彦式の開区間
cf. https://twitter.com/taketo1024/status/967767145584123904