@article{AIHPA_1994__60_4_373_0,
     author = {Anderson, W. G. and McLenaghan, R. G.},
     title = {On the validity of {Huygens'} principle for second order partial differential equations with four independent variables. {II.} {A} sixth necessary condition},
     journal = {Annales de l'I.H.P. Physique th\'eorique},
     pages = {373--432},
     publisher = {Gauthier-Villars},
     volume = {60},
     number = {4},
     year = {1994},
     mrnumber = {1288586},
     zbl = {0806.35104},
     language = {en},
     url = {http://archive.numdam.org/item/AIHPA_1994__60_4_373_0/}
}