1. School of Mathematics and Statistics; Southwest University; Chongqing 400715 People's Republic of China
2. Centro de Modelamiento Matemático (UMI CNRS 2807); Universidad de Chile; Beauchef 851 Santiago Chile
3. Department of Mathematics; Faculty of Sciences of Tunis; University of Tunis El Manar; Campus Universitaire 2092 Tunis Tunisia
4. Department of Mathematical Sciences; University of Bath; Bath BA2 7AY United Kingdom
5. Departamento de Matemáticas; Universidad Católica de Chile; Macul 782-0436 Chile