ACKNOWLEDGMENTS

We would like to thank our students and colleagues for encouraging us to write this book, and other people for various contributions and help: in particular, Jarred Tanner for providing several solutions to exercises in the original Kreiss notes, Barbro Kreiss for being of invaluable help, and Gunilla Kreiss for carefully reviewing the manuscript and making suggestions.

This book was written partially with the support of grants 05/B454 from SeCyT, Universidad Nacional de Córdoba, and the Partner Group grant of the Max Planck Institute for Gravitational Physics, Albert-Einstein-Institute (Germany).

H.-O.K. and O.E.O.