My greatest thanks go to my mathematics teacher Dr. Franz Hagen, who accompanied me from the 9th grade to the Abitur (final high-school examination) at the Gymnasium Oer-Erkenschwick (high-school). He was the one who gave me the joy of mathematics and recognized something like a talent. I dedicate this website to him.

I thank the many colleagues all over the world who were so kind to answer my questions, to criticize my texts and ideas constructively or to help me in other ways. Furthermore, I would like to mention the following people in alphabetical order.