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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?

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Look at the accents package. Welcome to TeX.sx! A tip: you can use backticks ` to mark your inline code as I did in my edit. – egreg Jul 14 '11 at 22:39
possible duplicate of Sub tilde in latex – Leo Liu Jul 15 '11 at 5:57
The answers can be merged to Sub tilde in latex – Leo Liu Jul 15 '11 at 5:58

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

This is $\frac{\utilde{ab}}{c} \neq \frac{ab}{c}$ and so on.

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

This is $\underaccent{\tilde}{\mathcal{A}}$ and so on.

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

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It seems that \usepackage{under tilde} is unavailable in TeXLive, only in MiKTeX. – Yrogirg Jul 18 '15 at 10:30
The solution can be found in tex.stackexchange.com/questions/220626/… – Yrogirg Jul 18 '15 at 10:39

Using \underset:


...or \stackrel:

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