Abstract: | The present paper is an abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Physical and Mathematical Sciences. The dissertation was defended on November 30, 1967 before the faculty of the V. A. Steklov Mathematical Institute of the Academy of Sciences of the USSR. The official opponents were Professors N. P. Korneichuk, P. P. Korovkin, and S. B. Stechkin, Doctors of Physical and Mathematical Sciences. |