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First of all, I would like to express my deep and sincere appreciation to Prof. Dr. W. Freeden, for his continuous guidance and help, as well as for the stim-ulating discussions we had during the preparation of this thesis. Without his continuous support and comments the completion of this thesis would have been impossible. Further, I am grateful to Prof. Dr. M. Schreiner for his valuable advice, great enthusiasm and interaction over the years. Special thanks go to all former and present colleagues of the Geomathematic