Footnote inside stackrel

Question

I am writing an equation and I'd like to put above the equal sign $(=)$ a footnote describing shortly the reason for that equality.

I tried

\[x^2\stackrel{\footnote{foo}}{=}x\ast x\]

but this only shows the footnote's number above the equal sign but no foo on the bottom of the page.

Any idea? What is the best way to do this kind of trick?

Answer 1

Use \footnotemark inside and \footnotetext outside.

\documentclass{article}
%\usepackage{amsmath}
\begin{document}      
  \[
   x^2\stackrel{\footnotemark}{=}x\ast x
  \]
  \footnotetext{foo}
\end{document}

Answer 2

I don't think it's really a good idea; the strange number, coming from nowhere, above the equals symbol will hardly be interpreted as a footnote marker.

Footnote markers in math formulas should be avoided because they can get confused with exponents. Why not simply add the explanation beside the formula?

\[
x^2 = x * x \qquad\text{(by definition)}
\]

(recall that \text requires amsmath). If the explanation is too long it can go after the formula or, if you really prefer footnotes, the footnote marker can be added in the "short explanation".