Donetsk AndryushkovaNadezhdaPetrovna Donetsk National University, 24, University St., Donetsk, 283001, Donetsk Peoples Republic, Ukraine andruchkova@rambler.ru Read more about Andryushkova Nadezhda Petrovna UmanetsSergeyFedorovich Donetsk National University, 24, University St., Donetsk, 283001, Donetsk Peoples Republic, Ukraine umanecsf@mail.ru Read more about Umanets Sergey Fedorovich
