1. Physics Department "E. R. Caianiello", University of Salerno, I–84081, Italy
2. International Center for Relativistic Astrophysics, I.C.R.A., University of Rome La Sapienza, I–00185 Roma, Italy
3. Istituto per Applicazioni della Matematica C.N.R., Naples, I–80131, Italy
4. Physics Department, University of Rome La Sapienza, I–00185 Roma, Italy