I would like to thank my supervisor Dr. Maxim Lobashov for his
patience and continuous guidance throughout my work. Moreover, I am
very grateful to Dr. Ursula Kluwick, who helped me to correct
misspellings and deficiencies in my English.
Finally, I would like to thank my wife Veronika, my children Anna,
Maria and Christina and my parents, who supported me throughout my
work and gave me the time I needed to complete my work.