15 Acknowledgments
I am grateful to Peter Aichelburg, Herbert Balasin, Robert Bartnik, Robert Beig, Chiang-Mei Chen,
Piotr ChruĊciel, Sergio Dain, Jörg Frauendiener, Sean Hayward, Jacek Jezierski, Jerzy Kijowski, Stephen
Lau, Lionel Mason, Niall Ó Murchadha, James Nester, Ezra Newman, Alexander Petrov, Walter Simon,
George Sparling, Paul Tod, Roh Tung, and Helmuth Urbantke for their valuable comments, remarks, and
stimulating questions. Special thanks to Jörg Frauendiener for continuous and fruitful discussions in the
last eight years, to James Nester for the critical reading of an earlier version of the present manuscript,
whose notes and remarks considerably improved its clarity, and to the two referees whose constructive
criticism helped to make the present review more accurate and complete. Thanks are due to the
Erwin Schrödinger Institute, Vienna, the Stefan Banach Center, Warsaw, the Universität
Tübingen, the Max-Planck-Institut für Mathematik in den Naturwissenschaften, Leipzig,
the National Center for Theoretical Sciences, Hsinchu, and the National Central University,
Chungli, for hospitality, where parts of the present work were done and/or could be presented.
This work was partially supported by the Hungarian Scientific Research Fund grant OTKA
T042531.