  We prove theorems about the Ricci and the Weyl tensors on generalized
Robertson-Walker space-times of dimension $n\ge 3$. In particular, we show that
the concircular vector introduced by Chen decomposes the Ricci tensor as a
perfect fluid term plus a term linear in the contracted Weyl tensor. The Weyl
tensor is harmonic if and only if it is annihilated by Chen's vector, and any
of the two conditions is necessary and sufficient for the GRW space-time to be
a quasi-Einstein (perfect fluid) manifold. Finally, the general structure of
the Riemann tensor for Robertson-Walker space-times is given, in terms of
Chen's vector. A GRW space-time in n = 4 with null conformal divergence is a
Robertson-Walker space-time.
