How to put a tilde under the symbol in math mode?

Question

It is very easy to put a tilde over the symbol in math mode. Simply type $\tilde{a}$. But how to put a tilde under the symbol in math mode?

Answer 1

The undertilde package provides extensible tilde accents under elements in math mode via \utilde{<symbols>}. Try, for example:

\documentclass{minimal}
\usepackage{undertilde}
\begin{document}
This is $\frac{\utilde{ab}}{c} \neq \frac{ab}{c}$ and so on.
\end{document}

Alternatively, as @egreg suggests, the accents package provides \underaccent[<accent>]{<symbol>}. Try, for example:

\documentclass{minimal}
\usepackage{accents}
\begin{document}
This is $\underaccent{\tilde}{\mathcal{A}}$ and so on.
\end{document}

See page 2 of the package documentation in order to extend this using \widetilde rather than \tilde. It depends on your preference.

Answer 2

Using \underset:

\underset{\sim}{=}

...or \stackrel:

\stackrel{=}{\sim}