1. Department of Mathematics; University of Pannonia; Veszprem Hungary
2. Department of Mathematics, State Key Lab of CAD & CG; Zhejiang University; Hangzhou 310027 People's Republic of China
3. Alfréd Rényi Institute of Mathematics, Hungarian Academy of Sciences; Budapest Hungary
4. Department of Computer Science and Systems Technology; University of Pannonia; Veszprém Hungary
5. Department of Mathematics; Zhejiang University; Hangzhou 310027 People's Republic of China
6. Department of Electrical Engineering and Information Systems; University of Pannonia; Veszprém Hungary