Volgograd MelnichenkoDmitryV. Volgograd State University, 400062, Volgograd region, Volgograd, University Avenue, 100melnichenko-dmi@mail.ru Read more about Melnichenko Dmitry V. GavrilovSergeyD. Volgograd State University, 400062, Volgograd region, Volgograd, University Avenue, 100gavrilov_sd@volsu.ru Read more about Gavrilov Sergey D. FantrovP.P. Volgograd State University, 400062, Volgograd region, Volgograd, University Avenue, 100pavelfantrov@rambler.ru Read more about Fantrov P. P.