1. Dpto. Ecuaciones Diferenciales y Análisis Numérico Universidad de Sevilla, Apdo. de Correos 1160, 41080-Sevilla, Spain
2. Department of Applied Mathematics Illinois Institute of Technology, Chicago, IL 60616, USA
3. Department of Mathematics Brigham Young University, Provo, Utah 84602, USA
4. Institut für Mathematik Universität Paderborn, 33098 Paderborn, Germany