Author(s):
1. Ivana Radonjić, 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. Dragana Milosavljević, University of Nis, Faculty of Sciences and Mathematics, , Serbia
5. Lana Pantić, University of Nis, Faculty of Sciences and Mathematics, , Serbia