[Tex/LaTex] Undefined control sequence with \overbar

Question

can anyone tell me why I get an "undefined control sequence" error with the following code please?

\documentclass[times]{fldauth}

\usepackage{amsmath}
\usepackage{amsthm, amssymb, amsfonts}

\begin{document}

\begin{equation}
\frac{\partial \overbar{u_i}}{\partial x_i}=0
\label{eq1n1}
\end{equation}

\end{document}  

Answer 1

\overbar is wrong initially (I did not found fldauth.cls)

This shows the (incomplete) possibilities to use a \overline - like typesetting of a mathematical symbol

\documentclass{article} %[times]{fldauth}

\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{mathabx}
\begin{document}

%\begin{equation}
%\frac{\partial \overbar{u_i}}{\partial x_i}=0
%\label{eq1n1}
%\end{equation}

\begin{align}
  \dfrac{\partial \bar{u_i}}{\partial x_i} &=0 \\
  \dfrac{\partial \widebar{u_i}}{\partial x_i} &=0 \\
  \dfrac{\partial \overline{u_i}}{\partial x_i} &=0 
\end{align}

\end{document}