Acknowledgement

First of all I have to thank my supervisor Marcel Lippmann. I had a lot of fruitful dis-cussions with him, where he was always interested in my thoughts. His critical questions where more than once leading to an improved understanding of the subject and working with him was not only helpful, it has been a pleasure. I am also grateful to Professor Franz Baader and Professor Bernhard Ganter. Their en-gagement made it possible to write this thesis on the faculty of informatics. A special thanks also goes to the Technische Universität Dresden for making this whole great studies of mathematics, including one unforgettable year abroad, possible. I partic-ularly wish to thank Professor Franz Baader and Professor Horst Reichel from the depart-ment of informatics and Professor Reinhard Pöschel and Professor Bernhard Ganter from the department of mathematics whose inspiring lectures and seminars had great influence on my studies. There are not so many friends of mine that enjoy talking about my thesis. Marc Häfner was more than helpful with his questions and thoughts. During this whole period of my studies he was always listening, discussing and cheering me up