\begin{thebibliography}{1}

\bibitem{Otto99b}
{\sc K.~Otto}, {\em Iterative solution of the {H}elmholtz equation by a
  fourth-order method}, Boll. Geof. Teor. Appl., 40:1 suppl. (1999),
  pp.~104--105.

\bibitem{KaOtto00}
{\sc I.~Karasalo and K.~Otto}, {\em A numerical study of the acoustic field in
  a fiber-optic hydrophone}, Meth. Rep. FOA-R--00-01780-409--SE, Division of
  Sensor Techno\-logy, Swedish Defence Research Agency, Lin\-k{\"o}p\-ing,
  Swe\-den, 2000.

\bibitem{KaOtto01}
\leavevmode\vrule height 2pt depth -1.6pt width 23pt, {\em Acoustic scattering
  from submerged elastic objects}, Meth. Rep. FOI-R--0307--SE, Division of
  Systems Techno\-logy, Swedish Defence Research Agency, Stock\-holm, Swe\-den,
  2001.

\bibitem{Otto03}
{\sc K.~Otto}, {\em A tensor framework for preconditioners based on fast
  transforms}, BIT,  (submitted 2003).

\end{thebibliography}
