2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/148564An analytic expression is given for the growth function for linear perturbations in a low-density universe made flat by a cosmological constant. The result involves elliptic integrals but is otherwise short and straight-forward.5 pages, LaTeX, 1 ps fig included. Submitted to ApJ. No text changes; LaTeX altered to be backwards compatible with v2.09Astrophysicstext