1. Department of Applied Mathematics, Donghua University, Shanghai 201620, P.R. China. E-mail: yuming_qin@hotmail.com
2. School of Information Science and Technology, Donghua University, Shanghai 201620, P.R. China
3. College of Information and Management Science, Henan Agricultural University, Zhengzhou 450046, P.R. China. E-mail: keqinsu@hotmail.com