1. Assistant Professor, Dept. of Mathematics, School of Mathematics and Statistics, Xi’an Jiaotong Univ., Xi’an 710049, P.R. China.
2. Professor, Dept. of Mechanics and Engineering Science, School of Mathematics, Jilin Univ., Changchun 130012, P.R. China (corresponding author).
3. Professor, Dept. of Mechanics and Engineering Science, School of Mathematics, Jilin Univ., Changchun 130012, P.R. China.