该yhmath
包定义了一个可扩展的\wideparen
数学重音:
\documentclass[a4paper, 11pt]{book}
\usepackage[utf8]{inputenc}
\usepackage{yhmath}
\begin{document}
\[ \wideparen{\mathbf{j}}\neq \wideparen{\wideparen{\mathbf{j}_e }}\neq \wideparen{\wideparen{\mathbf{j}_{e ,f}}}\]%
\end{document}