In the modern theory of boundary value problems the following ap- proach to investigation is agreed upon (we call it the functional approach): some functional spaces are chosen; the statements of boundary value prob- the basis of these spaces; and the solvability of lems are formulated on the problems, properties of solutions, and their dependence on the original data of the problems are analyzed. These stages are put on the basis of the correct statement of different problems of mathematical physics (or of the definition of ill-posed problems). For example, if the solvability of a prob- lem...