Closed Differential Form

\[ \newcommand{\d}{\mathrm{d}} \newcommand{\e}{\mathrm{e}} \newcommand{\i}{\mathrm{i}} \]

A smooth differential form \(\omega\) is closed, iff

\begin{equation*} d\omega=0, \end{equation*}

where \(d\omega\) denotes the exterior derivative of \(\omega\).