\begin{thebibliography}{1a}

\bibitem[1a]{HeOtto96}
{\sc L.~Hemmingsson and K.~Otto}, {\em Analysis of semi-{T}oeplitz
  preconditioners for first-order {PDE}s}, SIAM J.~Sci. Comput., 17 (1996),
  pp.~47--64.

\bibitem[1b]{Otto96}
{\sc K.~Otto}, {\em Analysis of preconditioners for hyperbolic partial
  differential equations}, SIAM J.~Numer. Anal., 33 (1996), pp.~2131--2165.

\bibitem[1c]{Hemm98}
{\sc L.~Hemmingsson}, {\em A semi-circulant preconditioner for the
  convection-diffusion equation}, Numer. Math., 81 (1998), pp.~211--248.

\bibitem[2a]{BrHoSu07}
{\sc H.~Brand{\'e}n, S.~Holmgren, and P.~Sundqvist}, {\em Discrete fundamental
  solution preconditioning for hyperbolic systems of {PDE}}, J.~Sci. Comput.,
  30 (2007), published electronically in December 2005.

\bibitem[2b]{AxNey06}
{\sc O.~Axelsson and M.~Neytcheva}, {\em Eigenvalue estimates for
  preconditioned saddle point matrices}, Numer. Linear Algebra Appl., 13
  (2006), pp.~339--360.

\bibitem[3a]{Larsson99}
{\sc E.~Larsson}, {\em A domain decomposition method for the {H}elmholtz
  equation in a multilayer domain}, SIAM J.~Sci. Comput., 20 (1999),
  pp.~1713--1731.

\bibitem[3b]{OttoLa99}
{\sc K.~Otto and E.~Larsson}, {\em Iterative solution of the {H}elmholtz
  equation by a second-order method}, SIAM J.~Matrix Anal. Appl., 21 (1999),
  pp.~209--229.

\bibitem[3c]{BaNey05b}
{\sc E.~B{\"a}ngtsson and M.~Neytcheva}, {\em Numerical simulations of glacial
  rebound using preconditioned iterative solution methods}, Appl. Math., 50
  (2005), pp.~183--201.

\end{thebibliography}