1. School of Information Science and Engineering; Chengdu University; Chengdu 610106 P.R. China
2. Department of Computer and Information Science; University of Macau; Macau P.R. China
3. Department of Applied Mathematics; University of Waterloo; Waterloo Ontario Canada
4. School of Mathematical Sciences; University of Electronic Science and Technology of China; Chengdu 611731 P.R. China