[Tex/LaTex] insert a symbol above a letter

accentsequationssymbols

How do I draw a curve accent over a letter in math mode, as shown below?

enter image description here

Best Answer

The yhmath package defines an extensible \wideparen math accent:

 \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} 

enter image description here