Author(s):
1. Dragana Todorović, University of Nis, Faculty of Sciences and Mathematics, , Serbia
2. Tomislav Pavlović, University of Nis, Faculty of Sciences and Mathematics, , Serbia
3. Dragoljub Mirjanić, Akademija nauka i umjetnosti Republike Srpske, Republic of Srpska, Bosnia and Herzegovina
4. Aleksandar Radivojević, University of Nis, Faculty of Sciences and Mathematics, , Serbia

Abstract