1. Faculty of Technical Sciences, Institute of Mechanics, University of Novi Sad, Trg D. Obradovića 6, 21000 Novi Sad, Serbia
2. Institute of Mathematics, Serbian Academy of Sciences and Arts, Kneza Mihaila 36, 11000 Belgrade, Serbia
3. Faculty of Sciences, Department of Mathematics and Informatics, University of Novi Sad, Trg D. Obradovića 4, 21000 Novi Sad, Serbia