Although certain important theoretical advances are described in some detail, especially in chapters 4,5, and 6, the book is intended primarily for users, not for numerical analysts. As in other parts of mathematics, the abstract theory and applications have tended to grow apart, and a new branch of pure mathematics has come into being in this area. Certain parts of the theory that are useful or significant for practical applications are presented, and the practical approach that was emphasized in the first edition has been retained.
