1. Istituto per le Applicazioni del Calcolo "M. Picone", C.N.R., I-00161 Roma, Italy
2. ICRA International Center for Relativistic Astrophysics, University of Rome, I–00185 Rome, Italy
3. Department of Physics "E.R. Caianiello", University of Salerno, I-84081, Italy
4. Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth, PO1 2EG, UK
5. Department of Mathematical Sciences, Villanova University, Villanova, PA 19085, USA
6. Department of Physics, University of Rome, I-00185, Italy