Cosmological models in semi-classical and higher order gravitational theories

We have studied semiclassical models with a classical scalar field, giving exact solutions in the cases of a lambda phi (4) and an exponential self-in teraction potential, in the last case we have also studied the influence of the vacuum polarization terms on the stability of the power-law solutions. We have also found cosmological exact solutions to the higher order gravit ational equations derived from a Lagrangian with a R square (k) R structure , and investigated the stability of the de Sitter and Minkowski space-time in the sixth order approximation of this theory.