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'institut Henri Poincaré. Section A, Physique Théorique, Tome 21 (1974) no. 4, p. 347.
complète On the validity of Huygens' principle for second order partial differential equations with four independent variables. Part I : derivation of necessary conditions 
@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'institut Henri Poincar\'e. Section A, Physique Th\'eorique},
     pages = {347--347},
     publisher = {Gauthier-Villars},
     volume = {21},
     number = {4},
     year = {1974},
     mrnumber = {369926},
     zbl = {0304.35063},
     language = {en},
     url = {http://archive.numdam.org/item/AIHPA_1974__21_4_347_0/}
}
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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'institut Henri Poincaré. Section A, Physique Théorique, Tome 21 (1974) no. 4, p. 347. http://archive.numdam.org/item/AIHPA_1974__21_4_347_0/