how do i write this letter on latex (showed in picture)

Question

I am trying to write an equation in my thesis, but i have been looking for around 2 hours the way to write this symbol, hope somebody can help me.

Answer 1

Here it is, using the accent package:

\documentclass{minimal}
\usepackage{accents}
\begin{document}
$\underaccent{\tilde}{\mathcal{A}}$
\end{document}