Е.I. Shkelev – Dr.Sc. (Phys.-Math.), Professor, Lobachevsky State University of Nizhni Novgorod E-mail: shkelev@rf.unn.ru

А.V. Shirkaev – Post-graduate Student, Lobachevsky State University of Nizhni Novgorod;  Research-Engineer, Sedakov Scientific Research Institute of Measuring Systems E-mail: avshirkaev@mail.ru