1. University of Banja Luka, Faculty of Natural Sciences and Mathematics, Mladena Stojanovića 2, 78000 Banja Luka, Bosnia and Herzegovina
2. University of Ljubljana, Faculty of Electrical Engineering, Tržaška cesta 25, 1000 Ljubljana, Slovenia
3. Institute of Mathematics, Physics and Mechanics, Department of Mathematics, Jadranska 19, 1000 Ljubljana, Slovenia