1. Zdzisław A. Golda, Andrzej Woszczyna, Karolina Zawada
Canonical gauge-invariant variables
Acta Phys. Pol., B , vol. B36, p. 2133 (2005).
[abstract] [preprint] [journal]

Abstract:
Under an appropriate change of the perturbation variable Lifshitz-Khalatnikov propagation equations for the scalar perturbation reduce to d'Alembert equation. The change of variables is based on the Darboux transform