1. University of Granada, Departamento de Matemática Aplicada, 18071-Granada, Spain
2. Politecnico of Torino, 10129-Torino, CNR IMATI, Pavia, Italy
3. School of Mathematical Sciences, Shanghai Jiao Tong University, Shanghai 200240, P. R. China
4. Institut für Mathematik, Universität Paderborn, 33098 Paderborn, Germany