2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/178509We construct a cosmological toy model in which a step-function ``cosmological constant'' is taken into consideration beside ordinary matter. We assume that $Λ$ takes two values depending on the epoch, and matter goes from a radiation dominated era to a dust dominated era. The model is exactly solvable and it can be compared with recent observations.9 pages, Latex, submitted to Phys. Lett. AAstrophysicstext