Affiliation: | 1. Ural Federal University, Institute of Natural Science and Mathematics, 620002, 19 Mira Str., Ekaterinburg, Russia;2. University of Agder, School of Business and Law, Department of Economics and Finance, Servicebox 422, N-4604 Kristiansand S, Norway;3. Institute of Mathematics, NASU, 3 Tereshchenkivska Str., 01601 Kiev, Ukraine |