Affiliation: | 1. Departamento de Ing. Matemática, F.C.F.M. Universidad de Chile, Casilla 170, Correo 3, Santiago, Chile;2. Departamento de Matemática, Universidad Técnica Federico Santa María, Casilla V-110, Avda. España 1680, Valparaíso, Chile;3. Department of Mathematics, Michigan State University, East Lansing, MI 48824, USA;4. College of Mathematics and Information Science, Guangzhou University, Guangzhou 510006, PR China |