On the validity of Huygens' principle for second order partial differential equations with four independent variables. II. A sixth necessary condition W. G. Anderson; R. G. McLenaghan — 1994 Annales de l'I.H.P. Physique théorique
Huygens' principle for the non-self-adjoint scalar wave equation on Petrov type III space-times W. G. Anderson; R. G. McLenaghan; F. D. Sasse — 1999 Annales de l'I.H.P. Physique théorique