Acknowledgements
I would like to acknowledge my indebtedness and render my warmest thanks
to my supervisor, Professor Arto Salomaa, who made this work possible.
His friendly guidance and expert advice have been invaluable throughout
all stages of the work.
I would also wish to express my gratitude to Professor Grzegorz Rozen-
berg for extended discussions and valuable suggestions which have con-
tributed greatly to the improvement of the thesis.
The thesis has also benefited from comments and suggestions made by
Dr. Gheorghe P˘aun and Dr. Alexandru Mateescu who have read through
the manuscript. I take this opportunity to thank them.
Special thanks are due to my husband, Dr. Jarkko Kari, for his con-
tinuous support and understanding, but also for more concrete thinks like
commenting on earlier versions of the thesis, helping with the figures and
the final preparation of the manuscript. My thanks are extended to my
father-in-law, Principal Antti Kari, for the drawing on the cover.
The person with the greatest indirect contribution to this work is my
mother, Dumitra Sˆantean, who has taught me love of mathematics. I want
to thank her, my father, as well as my brother Nic, for their constant
encouragement.
This thesis has been written during my stay at the Mathematics De-
partment of the University of Turku. I would like to thank the Academy of
Finland for providing excellent working conditions and Suomen Kulttuuri-
rahasto (Maili Autio foundation) for its financial support.
Turku, September 30th, 1991
Lila Kari
