1. Department of Mathematics, Northwest Normal University 1 , Lanzhou, Gansu Province 730070, People’s Republic of China
2. Department of Mathematics, Wenzhou University 2 , Wenzhou, Zhejiang Province 325035, People’s Republic of China
3. Departmento de Ecuaciones Diferenciales y Análisis Numérico, Facultad de Matemáticas, Universidad de Sevilla 3 , c/Tarfia s/n, 41012 Sevilla, Spain