1. Department of Mathematics, Harbin Institute of Technology, Weihai, Shandong 264209, P.R.C.; School of Computer Science and Technology, Yantai University, Yantai, Shandong 264005, P.R.C.
2. School of Computer Science and Technology, Yantai University, Yantai, Shandong 264005, P.R.C.