1. Department of Mathematical Modeling, Kaunas University of Technology, Studentu 50, 51368 Kaunas, Lithuania and Department of Finance, VSB-Technical University Ostrava, Faculty of Economics, Sokolská trida 33, 702 00 Ostrava 1, Czech Republic
2. Department of Mathematical Modeling, Kaunas University of Technology, Studentu 50, 51368 Kaunas, Lithuania