\documentclass{rtaloop}
\rtalabel{QPM}


\begin{document}
\begin{problem}{Richard Statman}{}{June 1993}

\begin{abstract}
In the $\lambda$-calculus, which sets have the form $\{M | Q \rightarrow^*
PM\}$? 
\end{abstract}

It has been proved that (in $\lambda$-calculus or Combinatory Logic) every
recursively enumerable set of ground terms that is closed under conversion
has the form $\{M | PM \leftrightarrow^* Q\}$ for some $P$ and $Q$.
Which sets have the form $\{M | Q \rightarrow^* PM\}$?

\end{problem}
\end{document}
