I often use the stackrel command within an equation in the following way
\documentclass{article}
\begin{document}
%
\begin{equation}
e^{i\pi} + 1 \stackrel{\footnotemark}{=} 0
\end{equation}
\footnotetext{this is Euler's identity.}
%
\end{document}
I'd like to reduce the vertical distance between the footnotemark and the equal sign by default. How can I do it?

