1. Department of Mathematics and Statistics, University of Strathclyde, 26 Richmond Street, Glasgow G1 1XH, UK.
2. Department of Mathematical and Computational Methods, National Laboratory for Scientific Computing (LNCC), Av. Getulio Vargas 333, Petropolis RJ, Brazil.
3. Departamento de Ingeniería Matemática and CI²MA, Universidad de Concepción, Concepción 4089100, Chile.