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}

Answer 3

We can define a macro:

\def\underacc #1/#2{\mathchoice{\uacc\textstyle{#1}{#2}}{\uacc\textstyle{#1}{#2}}
                     {\uacc\scriptstyle{#1}{#2}}{\uacc\scriptscriptstyle{#1}{#2}}}
\def\uacc#1#2#3{\mathop{#2{}}\limits_{#1#3{}}}

%test:
$\underacc a/\tilde, \tilde a$

Answer 4

Using «amssymb» package we can use macro such as \lesssim if we want to place '~' below '<'. Similarly \eqsim and \gtrsim if we want to place '~' below = or '>'

Answer 5

In 2021, the most convenient way is use the accents package

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