'Summarising this is a well-written book. It provides the complete survey of the history and the recent state of the subject topic. The book will be of use to postgraduate students and researchers in PDE's, in theory of approximations and expansions, in potential theory or in functional analysis. It is warmly recommended as a text for seminars and courses, as well as for independent study for everyone with a basic knowledge on distributions and pseudodifferential operators.' Acta Scientiarum Mathematicarum