[Tex/LaTex] How to properly typeset math intervals

Question

I have the code

\documentclass{amsart}

\begin{document}
    \begin{equation}
        ]-\pi,0[ \qquad -\pi \qquad \int_{-\pi}^0 \qquad [-\pi,0]
    \end{equation}
\end{document}

which produces

Why is the minus sign on the left so far away from pi and how can this be avoided?

Answer 1

In the TeXbook, Knuth refers to people using “]a,b[” notation for open intervals as “perverse mathematicians” (page 171, exercise 18.14). I don't fully agree with the adjective, but I find the notation very awkward nonetheless.

There is already a package for this, which avoids reinventing the wheel:

\documentclass{amsart}
\usepackage{interval}

\begin{document}

\begin{equation}
\interval[open]{-\pi}{0}
 \qquad
{-\pi} % this needs braces because it is between Ord atoms
 \qquad
\int_{-\pi}^0
 \qquad
\interval{-\pi}{0}
\end{equation}

\begin{equation}
x\in\interval[open]{-\pi}{0}
\end{equation}

\end{document}

The middle -\pi needs braces, but it's a very different problem.

See the documentation for interval to learn about other options.

---

Note about Werner's solution

Consider the following code and compile it to see what results:

$x \in ]{-\pi},0[$

$x \in \mathopen]-\pi,0\mathclose[$

(properly embed it in a standard document).

It should be clear that simply bracing -\pi is not sufficient.