Abstract:
Author's abstract of a thesis for the attainment of the academic degree of Doctor of Physics and Mathematics. The thesis was defended on April 25, 1968 at a meeting of the Scientific Council of the V. A. Steklov Mathematical Institute, Academy of Sciences of the USSR. Official opponents: Academicia A. N. Kolmogorov, Yu. V. Prokhorov, Corresponding Member of the Academy of Sciences of the USSR, and Prof. Yaglom, Doctor of Physics and Mathematics.