Add to ORKGI wish to express my sincere gratitude to my advisor, Professor Eero Saksman, for the admirable expert guidance I received from him throughout the work. I would also like to thank the people at the Department of the Mathematics and Statistics of the University of Jyväskyla ̈ for their great support during the past years. My special thanks go to Professor Juha Kinnunen, Daniel Aalto and Ville Suomala for inspiring discussions and encouragement they gave me. For the financial support I am indebted to the Graduate School in Mathe-matical Analysis and its Applications and the Vilho, Yrjo ̈ and Kalle Väisälä foundation. Finally, my most profound thanks are due to my loving family Riitta