A Time-dependent Cosmological Constant Phenomenology
  We construct a cosmological toy model in which a step-function ``cosmological constant'' is taken into consideration beside ordinary matter. We assume that $\Lambda$ 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. 
Salvatore Capozziello, Ruggiero de Ritis, Alma Angela Marino