[1] Juraj Dobrila University of Pula, Faculty of Engineering, Pula,52100, Croatia
[2] Mathematical Institute, The Serbian Academy of Sciences and Arts, Belgrade,11000, Serbia
[3] Juraj Dobrila University of Pula, Faculty of Informatics, Pula,52100, Croatia
[4] University of Rijeka, Faculty of Engineering, Rijeka,51000, Croatia