2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/200972We show that hydrodynamical and field approaches in theory of cosmological scalar perturbations are equivalent for a single medium. We also give relations between notations introduced by V. Lukash, J. Bardeen, J. Bardeen et al. and G. Chibisov and V. Mukhanov.8 pages, no figures, submitted to Astronomy ReportsAstrophysicstext