1. Department of Mathematics, Guangdong University of Education, Guangzhou, Guangdong, P.R. China.
2. Faculty of Mathematics and Computer Science, Babeş-Bolyai University, Cluj-Napoca, Romania
3. School of Computing and Engineering, University of Derby, Derby, United Kingdom
4. Department of Mathematics and Engineering Sciences, Hellenic Military Academy, Vari Attikis, Greece