1. Belgorod National Research University, Belgorod 308015, Russia
2. SouthWest State University, Kursk 305040, Russia
3. Department of Mathematics and Physics, Lappeenranta University of Technology, P.O. Box 20, FIN-53852 Lappeenranta, Finland
4. Ioffe Institute, 26 Politekhnicheskaya, St. Petersburg 194021, Russia