All linear representations of the Poincaré group up to dimension 8 Stephen M. Paneitz — 1984 Annales de l'I.H.P. Physique théorique
A quartic conformally covariant differential operator for arbitrary pseudo-Riemannian manifolds (Summary). Paneitz, Stephen M. — 2008 SIGMA. Symmetry, Integrability and Geometry: Methods and Applications [electronic only]