Grytczuk, Aleksander, Faculty of Mathematics, Computer Science and Econometrics, University of Zielona Góra, ul.Prof.Szafrana 4a, 65-516 Zielona Góra, Poland; and Jan PaweÅ‚ II Western Higher School of International Commerce and Finance, ul.Plac SÅ‚owiaÅ„ski 9, 65-069 Zielona Góra, Poland, Poland