V.I. Krivenkov − Ph.D. (Phys.-Math.). E-mail: krivenkov.v@gmail.com

A.P. Davydov - Ph.D. (Phys.-Math.), Assistant Professor, Department of Applied Mathematics and Informatics, Physical-Mathematical Faculty Magnitogorsk State Technical University n.a. G. I. Nosov. E-mail: ap-dav@yandex.ru