1. Faculty of Mathematics and Physics, Huaiyin Institute of Technology, Huai'an, People's Republic of China
2. College of Science, Hohai University, Nanjing, People's Republic of China
3. Faculty of Applied Sciences, Jilin Engineering Normal University, Changchun, People's Republic of China
4. Departamento de Matemática, Universidade Estadual de Campinas, IMECC, Campinas, Brazil