Thesis submitted for the degree of Doctor Philosophiœ Faculty of Mathematics and Natural Sciences University of Oslo Norway 2005 © Håkon Wium Lie, 1994-2005 This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License. Submitted 29th of March, 2005, as partial fulfillment of the degree Doctor Philosophiœ At the Faculty of Mathematics and Natural Sciences University of Oslo