@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'institut Henri Poincar\'e. Section A, Physique Th\'eorique},
     pages = {261--281},
     publisher = {Gauthier-Villars},
     volume = {2},
     number = {3},
     year = {1965},
     mrnumber = {184692},
     zbl = {0127.18304},
     language = {en},
     url = {http://archive.numdam.org/item/AIHPA_1965__2_3_261_0/}
}