\begin{thebibliography}{1}
\expandafter\ifx\csname url\endcsname\relax
  \def\url#1{\texttt{#1}}\fi
\expandafter\ifx\csname urlprefix\endcsname\relax\def\urlprefix{URL }\fi

\bibitem{Bjorstad:1991:SSO}
P.~E. Bj{\o}rstad, J.~Mandel, On the spectra of sums of orthogonal projections
  with applications to parallel computing, BIT 31 (1991) 76--88.

\end{thebibliography}