A spectrum generating nilpotent group for the relativistic free particle
Annales de l'institut Henri Poincaré. Section A, Physique Théorique, Tome 13 (1970) no. 2, pp. 129-136. 
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     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://www.numdam.org/item/AIHPA_1970__13_2_129_0/}
}
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Tilgner, Hans. A spectrum generating nilpotent group for the relativistic free particle. Annales de l'institut Henri Poincaré. Section A, Physique Théorique, Tome 13 (1970) no. 2, pp. 129-136. http://www.numdam.org/item/AIHPA_1970__13_2_129_0/

[1] H. Tilgner, A Class of Solvable Lie Groups and their Relation to the Canonical Formalism.

[2] D. Simms, Lie Groups in Quantum Mechanics. Springer. Lecture, Notes in Mathematics, 52, Berlin, 1968. | MR | Zbl

[3] C.-M. Edwards et J. Lewis, Twisted Group Algebras. I. Commun. math. Phys. , t. 13, 1969, p. 119-130. | MR | Zbl