[Tex/LaTex] Put some text under a relation symbol

Question

How to put some text under a binary operation symbol?

For example like this – the text "m" under the symbol "=".

About using \scriptscriptstyle. This command don't give the disired result. Because, space between "m" and "=" is too big. For example

    \documentclass{article}
    \usepackage{amsmath}
    \begin{document}
    $A \underset{m}{=} B$ $A \underset{\scriptscriptstyle{m}}{=} B$
    \end{document}

There is the same vertical size of formula in both cases.

Answer 1

You can do it with \underset. Of course if the text is wider than the = sign, you should use \clap, or \mathclap if it's a formula in order to ensure proper spacing of the = sign:

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\[ A \underset{m}{=} B \]%
\[ A \underset{\mathclap{m + n}}{=} B \]%

\end{document}