@article{AIHPA_1970__13_2_129_0,
     author = {Tilgner, Hans},
     title = {A spectrum generating nilpotent group for the relativistic free particle},
     journal = {Annales de l'institut Henri Poincar\'e. Section A, Physique Th\'eorique},
     pages = {129--136},
     publisher = {Gauthier-Villars},
     volume = {13},
     number = {2},
     year = {1970},
     mrnumber = {286389},
     language = {en},
     url = {http://archive.numdam.org/item/AIHPA_1970__13_2_129_0/}
}