@article{MM2_1987__59_155260,
     author = {Vieri Benci and Donato Fortunato},
     title = {A },
     journal = {Manuscripta mathematica},
     pages = {441--456},
     publisher = {mathdoc},
     volume = {59},
     year = {1987},
     zbl = {0616.58014},
     url = {http://geodesic.mathdoc.fr/item/MM2_1987__59_155260/}
}