2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/104440Under 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.LaTeX, 9 pagesGeneral Relativity and Quantum CosmologyCanonical gauge-invariant variables for scalar perturbations in synchronous coordinatestext