1. Department of Mathematics-ISEG and CEMAT-IST, 1200-781 Lisbon, Portugal
2. Department of Mathematics and CEMAT-IST, 1049-001 Lisbon, Portugal
3. Istituto per le Applicazioni del Calcolo-CNR, 00185 Roma, Italy
4. Department of Technology, University of Naples "Parthenope," Napoli, Italy