%0 Journal Article
%A Anderson, W. G.
%A McLenaghan, R. G.
%T On the validity of Huygens' principle for second order partial differential equations with four independent variables. II. A sixth necessary condition
%J Annales de l'I.H.P. Physique théorique
%D 1994
%P 373-432
%V 60
%N 4
%I Gauthier-Villars
%U http://archive.numdam.org/item/AIHPA_1994__60_4_373_0/
%G en
%F AIHPA_1994__60_4_373_0