1. Dept. of Applied Mathematics, Northwestern Polytechnical Univ., Xi'an, China; and College of Science, Guizhou Minzu Univ., Guiyang, CN (Corresponding author), e-mail: jennycaijing@163.com
2. Dept. of Applied Mathematics, Northwestern Polytechnical Univ., Xi'an, CN, e-mail: ymshi@nwpu.edu.cn
3. Dept. of Applied Mathematics, Northwestern Polytechnical Univ., Xi'an, CN.