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
Hyperbolic methods for Einstein's equations. Reula, Oscar A. (1998) Living Reviews in Relativity [electronic only]