1. School of Mathematics and Statistics, Henan University, Kaifeng 475004, P. R. China
2. College of Science, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China
3. Department of Applied Mathematics, Illinois Institute of Technology, Chicago, IL 60616, USA
4. Henan Key Laboratory of Earth System Observation and Modeling, Henan University, Kaifeng 475004, P. R. China