1. Dpto. Ecuaciones Diferenciales y Análisis Numérico; Universidad de Sevilla; Campus Reina Mercedes 41012-Sevilla, Spain
2. Department of Mathematics and Statistics; Auburn University; Auburn AL 36849 USA
3. School of Mathematics and Statistics; Huazhong University of Science & Technology; Wuhan 430074 China
4. Felix-Klein-Zentrum für Mathematik; TU Kaiserslautern, D-67663 Kaiserslautern Germany