  We prove that in Robertson-Walker space-times (and in generalized
Robertson-Walker spacetimes of dimension greater than 3 with divergence-free
Weyl tensor) all higher-order gravitational corrections of the Hilbert-Einstein
Lagrangian density $F(R,\square R, ... , \square^k R)$ have the form of perfect
fluids in the field equations. This statement definitively allows to deal with
dark energy fluids as curvature effects.
