Invariant Measures and Maximal L^2 Regularity for Nonautonomous   Ornstein-Uhlenbeck Equations
  We characterize the domain of the realization of the linear parabolic operator Gu := u_t + L(t)u (where, for each real t, L(t) is an Ornstein-Uhlenbeck operator), in L^2 spaces with respect to a suitable measure, that is invariant for the associated evolution semigroup. As a byproduct, we obtain optimal L^2 regularity results for evolution equations with time-depending Ornstein-Uhlenbeck operators. 
Matthias Geissert, Alessandra Lunardi