Tavzikh Ibragimovich Ibragimov,
Kazan Federal University,
18 Kremlyovskaya Str., Kazan, 420008, Russia,
Tavzikh.Ibragimov@ksu.ru.
Ravza E'kzamovna Kulsharipova,
Kazan Federal University,
18 Kremlyovskaya Str., Kazan, 420008, Russia,
rkulshar@mail.ru.