1. Department of Mathematics, Istituto “G. Castelnuovo”, University of Rome “La Sapienza”, Piazzale Aldo Moro, 5, 00185 Rome, Italy
2. Technical Institute “R. Rossellini”, Via della Vasca Navale, 58 CP., 00146 Rome, Italy
3. Naval Technical Institute “M. Colonna”, Via Salvatore Pincherle, 201 CP., 00146 Rome, Italy