To my parents

First, I would like to thank my thesis adviser Prof. Dr. Michael Voit for his invaluable support and for suggesting this interesting topic to me. He gave me guidance throughout the development of this thesis, as well as during my graduate and undergraduate studies, starting with the very first course in calculus. I am very thankful to Prof. Dr. Jeanette Woerner for the advice that she gave to me, both of a mathematical and of a professional nature. I wholeheartedly thank Silvetta for her love and inspiration. I am deeply indebted and grateful to my parents Natalia and Iakov for giving me un-conditional support throughout my life and encouragement to pursue my goals. Roditeli, ot vsego serdca blagodar vas za vaxe terpenie i podderku