1. Department of Mathematics, Juhász Gyula Faculty of Education, University of Szeged, Hattyas utca 10, H-6725 Szeged, Hungary
2. University of Prishtina, Faculty of Education, Department of Mathematics and Informatics, Avenue “Mother Theresa” no. 5, Prishtinë 10000, Kosovo
3. University of Zielona Góra, Faculty of Mathematics, Computer Science and Econometrics, ul. Szafrana 4a, 65-516 Zielona Góra, Poland