All the notations and definitions used in this volume are standard
and commonly used. The reader can find them, for example, in the
textbooks  and , in which all necessary theoretical background
can be found. However, to make the book consistent and to avoid
ambiguity, a list of notations and definitions is included.
We have borrowed freely from many textbooks, problem books
and problem sections of journals like the American Mathematical
Monthly, Mathematics Today (Russian) and Delta (Polish). A com-
plete list is given in the bibliography. It was beyond the authors'
scope to trace all original sources, and we may have overlooked some
contributions. If this has happened, we offer our sincere apologies.
We are deeply indebted to all our friends and colleagues from the
Department of Mathematics of Maria Curie-Sklodowska University
who offered stimulating suggestions. We have had many fruitful con-
versations with M. Koter-Morgowska, T.Kuczumow, W. Rzymowski,
S. Stachura and W. Zygmunt. Our sincere thanks are also due to
Professor Jan Krzyz for his help in preparing the first version of the
English manuscript. We are pleased to express our gratitude to Pro-
fessor Kazimierz Goebel for his encouragement and active interest in
the project. It is our pleasure to thank Professor Richard J. Libera,
University of Delaware, for his invaluable and most generous help
with the English translation and for all his suggestions and correc-
tions which greatly improved the final version of the book.
W. J. Kaczor, M. T. Nowak