[Tex/LaTex] How to draw a dot under a symbol in math mode

accentsmath-mode

I'm trying to find the equivalent to \d (or \textsubdot) for the math mode.

You can use the \underaccent command from the accents package. Something along these lines:

\documentclass{article}
\usepackage{amsmath}
\usepackage{accents}

\newcommand*\underdot[1]{%
\underaccent{\dot}{#1}}

\begin{document}
$\underdot{A}$
\end{document}