Hidden symmetries of M -theory and its dynamical realization. Nurmagambetov, Alexei J. (2008) SIGMA. Symmetry, Integrability and Geometry: Methods and Applications [electronic only]
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
Huygens' principle on Petrov type N space-times V. Wünsch (1994) Annales de l'I.H.P. Physique théorique