@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://www.numdam.org/item/AIHPA_1974__21_4_347_0/}
}