“The author masterfully introduces the uninitiated reader to this beautiful but also demanding subject of mathematics in a step-by-step, self-contained manner. Throughout the book, the reader is also lead to explore more advanced aspects of the problems studied. The topics treated, the structure and the presentation of this book are such that it would be excellent for class as well as seminar use.” (Vijay Gupta, European Mathematical Society, euro-math-soc.eu, Issue 115, March, 2020)“The monograph is well readable since the author provides many details and thus makes the presentation self-contained. The book will be a valuable addition to every mathematical library. It will serve as an invaluable source to students and professors interested in the subject. It is an important contribution to the mathematical literature.” (Helmut Maier, zbMATH, 2017)