xCoba differentiation for general function of a tensor

[Sergey Slizovskiy]

Imagine that I introduce a chart {t,x,y,u}
and I have a scalar field f[u[]] depending only on one coordinate.
  The partial derivatives in this chart do not know how to differentiate composite unknown function f,
but they do it correctly if I replace  f with any known function like Sqrt
     Are there ways to change such behaviour?  Is it made on purpose to evade some bugs?

 

Shall I better define f as a scalar tensor and write rules for it's derivatives?

[Jose]

Hi Sergey,

You need to declare the function f to be a "scalar function". Use
DefScalarFunction[ f ]. In xAct you have to declare all objects you
use, so that the system knows what to do with them when they are
found.

The numeric functions of Mathematica (including Sqrt for example) are
all declared in advance to be of type ScalarFunction.

Just in case, beware of the difference between scalar functions and
scalar fields. The brackets in the scalar field x[] denote 0 indices.
The outer brackets in f[ x[], y[], z[] ] denote argument dependencies,
as usual in Mathematica. The system knows the difference precisely
because of the declarations.

Cheers,
Jose.

[Sergey Slizovskiy]

Thank you very much, Jose!