[Tex/LaTex] Undefined control sequence while writing equation

Question

I am writing an equation in LYX and I get the error "Undefined control sequence"
With the message

"The control sequence at the end of the top line of your error message
was never \def'ed. If you have misspelled it (e.g., \hobx'), typeI'
and the correct spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined."

\batchmode
\makeatletter
\def\input@path{{C://Desktop}}
\makeatother
\documentclass[english]{book}
\renewcommand{\familydefault}{\rmdefault}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{geometry}
\geometry{verbose,tmargin=1.5cm,bmargin=2.5cm,lmargin=2.5cm,rmargin=2.5cm}
\setcounter{secnumdepth}{3}
\setcounter{tocdepth}{3}
\usepackage{setspace}
\doublespacing
\usepackage{babel}

\begin{document}
\begin{equation}
c=\alpha+\beta[\frac{1}{\varOmega}\int_{\varOmega}\frac{|\nabla\rho(r)|}  {\rho(r)}dr]^{1/2}
\end{equation}
\end{document}

How to correct that?

Answer 1

Or simply add amsmath package in preamble of your document:

\documentclass[english]{book}
\renewcommand{\familydefault}{\rmdefault}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{geometry}
\geometry{verbose,tmargin=1.5cm,bmargin=2.5cm,lmargin=2.5cm,rmargin=2.5cm}
\setcounter{secnumdepth}{3}
\setcounter{tocdepth}{3}
\usepackage{setspace}
\doublespacing
\usepackage{babel}
\usepackage{amsmath}%<-- added or {mathtools}

    \begin{document}
\begin{equation}
c=\alpha+\beta[\frac{1}{\varOmega}\int_{\varOmega}\frac{|\nabla\rho(r)|}  {\rho(r)}dr]^{1/2}
\end{equation}
    \end{document}

For my taste the nicer look of equation is obtained by

\begin{equation}
c=\alpha+\beta\left[\frac{1}{\varOmega}\int_{\varOmega}\frac{|\nabla\rho(r)|}  {\rho(r)}\mathrm{d}r\right]^{1/2}
\end{equation}

i.e., similarly as suggested @egred in his answer.