I would like to express my gratitude to several people who helped me during the years in which this thesis arose. First and foremost, I want to thank Christina Metzdorf for her great patience and care during those past years. This thesis would have been unthinkable without her cheerful nature and her support. In the same breath I would like to thank my parents Cornelia Konrad-Kaip and Michael Konrad. All this time, they supported me by never losing faith in me. I would also like to thank them for their great solidarity in good times as well as in hard times.Furthermore, I feel a deep gratitude towards my advisors Prof. Dr. Robert Denk and Prof. Dr. Jürgen Saal for not only being my mentors but also my friends. I would particularly like to thank them for many stimulating discussions and for bringing me into contact with actual research topics. I have also deeply enjoyed the collaboration with Prof. Dr. Robert Denk while organizing with him many of his lectures and weekly exercises. Furthermore, I would like to show my gratitude to Prof. Dr. Wolfgang Watzlawek who deeply influenced my basic approach to mathematics.Moreover, my thanks goes to the staff of the faculty of mathematics, especially Gerda Baumann, Rainer Janßen, and Gisela Cassola. Their uncomplicated and straightforward help was not only very pleasant but also extremely helpful throughout the past years.Last but not least, I would like to thank my friends Dr. Thilo Moseler, Tobias Nau, Johannes Schnur, Tim Seger, Dr. Olaf Weinmann and the other members of the PDE research group in F5 for the great atmosphere and also for the numerous discussions. During the final stage of this thesis Martin Saal and Alexander Schöwe helped tremendously when taking over my task of preparing the weekly exercises, a help that is very much appreciated. I would also like to thank Johannes Schnur and Tim Seger for using their precious time to read this thesis. Besides, my special thanks goes to Johannes for his constant encouragement and his 'Gute Laune Versicherung'.
