\begin{thebibliography}{Wir98}

\bibitem[Wir98]{Wi98}
Günther~J. Wirsching.
\newblock {\em The dynamical system generated by the $3n+1$ function}, volume
  1681 of {\em Lecture Notes in Mathematics}.
\newblock Springer-Verlag, Berlin, 1998.

\end{thebibliography}
