1. Department of Mathmatics and Physics, Hefei University, Hefei, Anhui, P. R. China
2. School of Computer Science and Technology, Anhui University, Hefei, Anhui, P. R. China
3. School of Computer and Information, Fuyang Normal College, Fuyang, Anhui, P. R. China
4. Department of Mathematics and Computer Science, University of Münster Einsteinstr. 62, Münster, Germany
5. Department of Computer and Information Science, Faculty of Science and Technology, University of Macau, Macau