1. Center for Nonlinear Studies and School of Mathematics, Northwest University, Xi’an 710069, People’s Republic of China
2. School of Mathematics and Statistics, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China
3. Center for Nonlinear Studies and School of Mathematics and Statistics, Ningbo University, Ningbo 315211, People’s Republic of China