@article{AIHPA_1974__20_2_153_0,
     author = {McLenaghan, R. G.},
     title = {On the validity of {Huygens'} principle for second order partial differential equations with four independent variables. {Part} {I} : derivation of necessary conditions},
     journal = {Annales de l'institut Henri Poincar\'e. Section A, Physique Th\'eorique},
     pages = {153--188},
     publisher = {Gauthier-Villars},
     volume = {20},
     number = {2},
     year = {1974},
     mrnumber = {361452},
     zbl = {0287.35058},
     language = {en},
     url = {http://www.numdam.org/item/AIHPA_1974__20_2_153_0/}
}