\listfiles
\documentclass{jarticle}
\usepackage{amsmath}
\usepackage{accents,undertilde}

\begin{document}

\begin{tabular}{ll}
\verb|accents.sty|&\verb|undertilde.sty|\\
\hline
$\undertilde{ABc}$ &$\utilde{ABc}$\\
\hline
This is $\frac{\undertilde{ab}}{c} \neq \frac{ab}{c}$ and so on.
&
This is $\frac{\utilde{ab}}{c} \neq \frac{ab}{c}$ and so on.\\
\hline
\end{tabular}

\end{document}
