@article{MAN_1990__287_1_164701,
     author = {Carlo Greco},
     title = {Periodic trajectories of a class of {Lorentz-metrics} of a time-dependent gravitational field.},
     journal = {Mathematische Annalen},
     pages = {515--522},
     publisher = {mathdoc},
     volume = {287},
     number = {1},
     year = {1990},
     zbl = {0681.53033},
     url = {http://geodesic.mathdoc.fr/item/MAN_1990__287_1_164701/}
}