# Dealing with covariant derivatives (xCoba)

I do not understand why the output of the two commands:

metric[-c, -d] (CD[d][RicciScalar[CD][]]) (CD[c][RicciScalar[CD][]]) // Simplify,

(CD[-c][RicciScalar[CD][]]) (CD[c][RicciScalar[CD][]]) // Simplify,


is different. The first one outputs a scalar function, as expected, whereas the second gives an expression

that I do not know how to contract.

• Can you provide a minimal version of your code that reproduces the error when run on a "clean" kernel? It may help in tracking down the problem. Oct 8, 2021 at 19:47