"Many excellent books in this field have been published. But this is the first book that describes systematically the former, although this object is very important and very interesting. And in the latter also this book contains a number of results which are not touched on in other books. Therefore the reviewer is sure that many results contained in it will be of great interest to readers." Mathematical Reviews
"This book is a well written introduction to the study of these questions. A large number of examples are given to illustrate the different phenomena studied and to show the necessity of the hypotheses in the theorems." Zentralblatt für Mathematik
"This book is a well written introduction to the study of these questions. A large number of examples are given to illustrate the different phenomena studied and to show the necessity of the hypotheses in the theorems." Zentralblatt für Mathematik