1. School of Mathematics and Statistics, Northwestern Polytechnical University, Xi’an, People’s Republic of China, Innovation Center, Northwestern Polytechnical University-Chongqing, Chongqing, People’s Republic of China, and Faculty of Mathematics, Kyu
2. Faculty of Mathematics, Kyushu University, Fukuoka, Japan
3. School of Mathematics and Statistics, Northwestern Polytechnical University, Xi’an, People’s Republic of China