Non-adiabatic perturbations in multi-component perfect fluids
  The evolution of non-adiabatic perturbations in models with multiple coupled
perfect fluids with non-adiabatic sound speed is considered. Instead of
splitting the entropy perturbation into relative and intrinsic parts, we
introduce a set of symmetric quantities, which also govern the non-adiabatic
pressure perturbation in models with energy transfer. We write the gauge
invariant equations for the variables that determine on a large scale the
non-adiabatic pressure perturbation and the rate of changes of the comoving
curvature perturbation. The analysis of evolution of the non-adiabatic pressure
perturbation has been made for several particular models.
