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?


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.


Using \underset:


...or \stackrel:


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