Volgograd MedvedevaNataliaMikhailovna Volgograd State University, 100, University prospekt, Volgograd, Russia natasha_medvedeva@volsu.ru, nmedv@mail.ru Read more about N.M. Medvedeva EgorovVladislavValerievich Volgograd State University, 100, University prospekt, Volgograd, Russia yegoroff_vv@mail.ru Read more about V.V. Egorov
