答案1
结合。\overbrace
\mathop
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
=\mathop{\overbrace{\idotsint}^{n-1}}\limits_{x_1^2+\dots+x_n^2\le1}
\]
\end{document}
也许有一些视觉上的修复:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
=\mathop{\,\,\overbrace{\!\!\idotsint\,}^{n-1}\!}\limits_{x_1^2+\dots+x_n^2\le1}
\]
\end{document}