1. School of Mathematics and Statistics, Southwest University 1 , Chongqing 400715, China
2. Departamento de Ecuaciones Diferenciales y Análisis Numérico, Facultad de Matemáticas, Universidad de Sevilla 2 , C/Tarfia s/n, 41012 Sevilla, Spain
3. Department of Mathematics, Wenzhou University 3 , Wenzhou, Zhejiang Province 325035, People's Republic of China