@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}, zbl = {0304.35063}, mrnumber = {369926}, language = {en}, url = {http://www.numdam.org/item/AIHPA_1974__21_4_347_0/} }