I tried ctensor with the Robertson-Walker metric.
Following the definition of Riemann curvature tensor in the document
of ctensor, it seems that the Ricci tensor given by maxima is negative
what it should be.
The document doesn't give explicitly the definition of the Ricci
tensor, and I only ever see one definition as in
http://mathworld.wolfram.com/RicciCurvatureTensor.html
Is there a bug or I missed something?