1. Institute of Systems Science, Northeastern University, Shenyang, Liaoning 110819, P. R. China
2. School of Science, Shenyang University of Technology, Shenyang 110870, P. R. China
3. Division of Math, Science and Technology, Nova Southeastern University, Fort Lauderdale, Florida 33314, USA
4. School of Science, Dalian Jiaotong University, Dalian 116028, P. R. China