Complete solution of Hadamard's problem for the scalar wave equation on Petrov type III space-times S. R. Czapor; R. G. McLenaghan; F. D. Sasse — 1999 Annales de l'I.H.P. Physique théorique
Consequences of the validity of Huygens' principle for the conformally invariant scalar wave equation, Weyl's neutrino equation and Maxwell's equations on Petrov type II space-times J. Carminati; S. R. Czapor; R. G. McLenaghan; G. C. Williams — 1991 Annales de l'I.H.P. Physique théorique