@article{AIHPA_1965__2_3_261_0,
     author = {Vu Thanh Khi\^et},
     title = {On the sign of energy of the plane gravitational waves},
     journal = {Annales de l'I.H.P. Physique th\'eorique},
     pages = {261--281},
     publisher = {Gauthier-Villars},
     volume = {2},
     number = {3},
     year = {1965},
     mrnumber = {184692},
     zbl = {0127.18304},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/AIHPA_1965__2_3_261_0/}
}