答案1
\documentclass{article}
\usepackage{amsmath}
\usepackage{accents}
\newcommand{\Underbar}[1]{\underaccent{\bar}{#1}}
\newcommand{\ubar}{\underbar u}
\begin{document}
\begin{equation}
\Underbar{\Underbar\varepsilon} =
\frac{1}{2} (\accentset{*}{\ubar})
\end{equation}
\end{document}