"Written by two distinguished specialists in both research and education, the book represents an excellent introduction to a subject of increasing power and importance in today's mathematics. It is designed for senior undergraduate and graduate students in mathematics, engineering, etc., but it should be welcome in the library of any person interested in operator theory and its applications." - F.-H. Vasilescu for AMS "The text is self-contained as it is possible, and contains a large number of exercises. It is highly offered to graduate and senior undergraduate students in mathematics." -- L. Gehér for Acta Sci. Math. (Szeged)