Contemporary methods for solving problems of mathematical physics are gaining increasing popularity. They are being used more and more in solving applied problems not only by professional inathematicians, but also by investigators and users working in other branches of science, engineering and technology. In order to make this book accessible not only to specialists, but also to graduate and post-graduate students, we give a complete account of notions and definitions which will be used in the sequel. The concepts and theorems presented below are of an auxiliary nature and are included for references rather than for primary study. For this reason the majority of statements are quoted without proofs. We will also cite bibliographical sources for further, more detailed, information.