Huygens' principle for the non-self-adjoint scalar wave equation on Petrov type III space-times

Anderson, W. G.; McLenaghan, R. G.; Sasse, F. D.

Annales de l'I.H.P. Physique théorique, Volume 70 (1999) p. 259-276

On the validity of Huygens' principle for second order partial differential equations with four independent variables. II. A sixth necessary condition

Anderson, W. G.; McLenaghan, R. G.

Annales de l'I.H.P. Physique théorique, Volume 60 (1994) p. 373-432