1. Faculty of Physics and Mathematics, Voronezh State Pedagogical University, 394043 Voronezh, Russia, E-mail: valerio-ob2000@mail.ru
2. Università degli Studi di Firenze, Via Santa Marta 3, 50139 Firenze, Italy, E-mail:zecca@unifi.it
3. Faculty of Physics and Mathematics, Voronezh State Pedagogical University, 394043 Voronezh, Russia, E-mail: marya.afanasowa@yandex.ru