1. Faculty of Science and Mathematics, Department of Computer Science, University of Niš, Niš, Serbia
2. Department of Economics, Division of Mathematics and Informatics, National and Kapodistrian University of Athens, Athens, Greece
3. School of Mathematics and Statistics, Yunnan University, Kunming, People's Republic of China
4. School of Mathematical Science, Harbin Normal University, Harbin, People's Republic of China