Acknowledgements

This paper would not have been possible without the help of many people. I am much in-debted to Prof. Dr. M. Denker for suggesting the problem and giving constant encouragement and inspiration. My special thanks go to Prof. Dr. M. Gordin who brought several important references and examples to my attention and whose collaboration resulted in a joint paper. Also I owe thanks to PD Dr. M. Rösler for her advise concerning the applications to hyper-groups and to PD Dr. U. Fiebig for taking the Koreferat. During my time as a Ph.D. student I held a scholarship of the graduate school ”Gruppen und Geometrie”, and I would like to thank for the financial support that was provided. Last but not least I want to thank my colleagues in the work-group for discussions, help with Latex and the general good working atmosphere