no. 4
Explicit solutions of □u=0 on the Friedmann-Robertson-Walker space-times
Annales de l'institut Henri Poincaré. Section A, Physique Théorique, Tome 35 (1981) no. 4, pp. 253-257. 
@article{AIHPA_1981__35_4_253_0,
     author = {Klainerman, S. and Sarnak, P.},
     title = {Explicit solutions of $\square u = 0$ on the {Friedmann-Robertson-Walker} space-times},
     journal = {Annales de l'institut Henri Poincar\'e. Section A, Physique Th\'eorique},
     pages = {253--257},
     publisher = {Gauthier-Villars},
     volume = {35},
     number = {4},
     year = {1981},
     mrnumber = {650132},
     zbl = {0499.58028},
     language = {en},
     url = {http://www.numdam.org/item/AIHPA_1981__35_4_253_0/}
}
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Klainerman, S.; Sarnak, P. Explicit solutions of $\square u = 0$ on the Friedmann-Robertson-Walker space-times. Annales de l'institut Henri Poincaré. Section A, Physique Théorique, Tome 35 (1981) no. 4, pp. 253-257. http://www.numdam.org/item/AIHPA_1981__35_4_253_0/

[1] A. Einstein, The Meaning of Relativity. | Zbl

[2] F. Friedlander, Wave Equations in Curved Space Time, Cambridge Monographs. | Zbl

[3] Fritz John, Partial Differential Equations, Springer-Verlag.

[4] S. Hawking and G. Ellis, The Large Scale Structure of Space Time, Cambridge Monographs. | Zbl

[5] P. Lax and R. Phillips, Scattering Theory for Automorphic Functions, Annals of Mathematical Studies, Princeton. | Zbl