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The evolution of spatially homogeneous and isotropic cosmological modelscontaining a perfect fluid with equation of state p=w\rho\ and a cosmologicalconstant \Lambda\ is investigated for arbitrary combinations of w and \Lambda,using standard qualitative analysis borrowed from classical mechanics. Thisapproach allows one to consider a large variety of situations, appreciatingsimilarities and differences between models, without solving the Friedmannequation, and is suitable for an elementary course in cosmology.

Qualitative study of perfect-fluid Friedmann-Lema\^{\i}tre-Robertson-Walker models with a cosmological constant