Institution: | a Faculty of Applied Mathematics and Mechanics, Voronezh State University, Universitetskaya pl. 1, 394006 Voronezh, Russia b Faculty of Mathematics, Voronezh State University, Universitetskaya pl. 1, 394006 Voronezh, Russia c Dipartimento di Energetica “S. Stecco”, Università degli Studi di Firenze, Via S. Marta 3, 50139 Firenze, Italy |