Acknowledgements

First and foremost, I thank my supervisor, Professor Yingbo Zhang, for her patient guidance and encouragement on mathematics during this three years, for her providing me many good chances to improve myself, and for her care not only for my study, but also for my life. I would also like to thank my co-supervisors, Professor Henning Krause, whose enthusiasm for mathematics influences me a lot. His suggestion for the topics and his many enlightening discussions make this thesis possible. I express my gratitude to the Asia-Link project “Algebras and Represen-tations in China and Europe”, which supported me during my several visits in Europe. I would further like to thank Professor Changchang Xi, Professor P