1. Department of Engineering, University Campus Bio-Medico of Rome, Unit of Nonlinear Physics and Mathematical Modeling, Via A. del Portillo 21, 00128 Rome, Italy
2. Mathematical Institute, University of Oxford, Woodstock Road, OX2 6GG Oxford, United Kingdom
3. International Center for Relativistic Astrophysics (I.C.R.A.), University Campus Bio-Medico of Rome, Rome, Italy