\begin{thebibliography}{Pap94}

\bibitem[GJ79]{Garey}
M.~R. Garey and D.~S. Johnson.
\newblock {\em Computers and Intractability: A Guide to the Theory of
  NP-Completeness}.
\newblock W. H. Freeman Company, San Francisco, 1979.

\bibitem[HS01]{Homer1}
Steven Homer and Alan~L. Selman.
\newblock {\em Computability and Complexity Theory}.
\newblock Springer-Verlag, 2001.

\bibitem[Pap94]{Papa1}
Christos~H. Papadimitriou.
\newblock {\em Computational Complexity}.
\newblock Addison-Wesley, New York, 1994.

\end{thebibliography}