\documentclass{ppex}
\nonstopmode
\usepackage [all]{xy}
\pagestyle{empty}
\setlength\textwidth{207.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage [all]{xy}
\[
\xymatrix{
 A & *+[F]{\sum_{i=n}^m {i^2}} \\
   & {\bullet} & D \ar[ul]
}\]
\end{document}