Remarque sur « R. G. McLenaghan : on the validity of Huygens principle for second order partial differential equations with four independent variables. I »
Annales de l'I.H.P. Physique théorique, Tome 21 (1974) no. 4, p. 347.
@article{AIHPA_1974__21_4_347_0,
     author = {Helgason, Sigurdur},
     title = {Remarque sur \guillemotleft{} R. G. McLenaghan : on the validity of Huygens principle for second order partial differential equations with four independent variables. I \guillemotright{}},
     journal = {Annales de l'I.H.P. Physique th\'eorique},
     pages = {347--347},
     publisher = {Gauthier-Villars},
     volume = {21},
     number = {4},
     year = {1974},
     zbl = {0304.35063},
     mrnumber = {369926},
     language = {en},
     url = {archive.numdam.org/item/AIHPA_1974__21_4_347_0/}
}
Helgason, Sigurdur. Remarque sur « R. G. McLenaghan : on the validity of Huygens principle for second order partial differential equations with four independent variables. I ». Annales de l'I.H.P. Physique théorique, Tome 21 (1974) no. 4, p. 347. http://archive.numdam.org/item/AIHPA_1974__21_4_347_0/