Acknowledgments

Leaning back and surveying this work, I am glad to admit that it is quite different from what I have anticipated when I started on it years ago, because this divergence reflects the influence of the people I have worked with. I owe much more than just my interest in λ-calculi and natural deduction to my Doktorvater Prof. Dr. Helmut Schwichtenberg. It is his scientific impetus that inspires his working group and so many fruitful discussions; the open-minded atmosphere that he creates is so valuable to all of us. In particular, I thank him for his confidence in my mathematical fidelity, and his stamina to endure the never-ending evolution of my confluence and normalization proofs. I hope that some of his constructive understanding of logic is mirrored in my work. Ralph Matthes is probably the person to blame most for this thesis: Without him as kind of mathematical alter ego, all my ideas would have collapsed in a woeful heap of counterexamples and errors. The basic techniques that served as a starting point for this thesis have been conceived jointly in an intellectual symbiosis, that I count among the most exciting experiences of my life