Acknowledgements

I want to dedicate this thesis to my Oma Mien, who died last December, but who would have loved to have seen me graduate. She was always interested in me and my studies, provided me with many books and liked to have discussions with me on all kinds of topics. She inspired me to do more.

I would like to thank Kiene for her many, very inspiring classes, for having patience with me during the writing process, for being strict sometimes and for believing in my talents: I learned a lot from you. I would also like to thank Sander van Maas for giving us extra lectures on difference theory. Opa, thank you for correcting my English and giving me comments. Furthermore I owe many thanks to my family who always supported me in whatever I decided to do. Daddy, thanks