xAct Tensor Computer Algebra

i have attached the note book and i want to obtain the standrd text book form the Eqm. Applying rules

Hi! xAct really needs a new release. Mathematica changes the versions of the mathlink communication

Hi everyone, I am considering a specific class of scalar-tensor Horndeski theories. I have obtained

Hi Thomas, Thanks. This fixed it. Best -Mariem On Friday, December 12, 2025 at 5:30:29 PM UTC-5

You have an error saying "TensorValues::invalid: Ricci[-a,-b] is not a valid expression for

When you define the metric, the associated Levi-Civita connection is defined (the one you denoted LCD

Hi Cynthia! I think you might have a missunderstanding about how xTerior works. xTerior allows to

Hi James, I think you have found a bug but not in xPert, but in LieD[v,CovD] when acting on densities

Section 8.1-8.11 of xTensorDoc gives the definition and examples. You can find that document here:

You can replace -g_{μν} with u_μ u_ν. For theta, you need to replace the derivative of the velocity

Hi , I am trying to use xAct to derive the second–order action for scalar perturbations in a non-

Hi everyone! I'm very new to xAct (and Mathematica in general) and I've been trying to learn

Dear Sasha, you have to use DefCovD[DD[-i], lie, {":", "D"}, FromMetric ->

Dear Thomas, Thank you very much for your quick reply. I had completely missed the SymManipulator

A bit, for example; I can write a term cd[-a][cd[b][A[a]]] in this way: DefTensor[F[-a,-b],M,

I am working with xPert and xPand and would like to re-express time derivatives as derivatives with

Hi everyone, I am pretty new to xAct, so maybe this question is really simple. I'm doing a

Hello everyone 😁 I had the same error and I fixed it using: (*Arreglo del problema 13.+ *) (*1.

I solved it now with the usual xPert method and replacing the variation of the matter Lagrangian by

Hey everyone, As you will see in the attached notebook. I'm trying to generate some DDIs using

Hi, Is there any way to work with metric variation in arbitrary d dimension? I have been looking into

First notice that //. rules apply "rules" repeatedly until there are no more possible

Here is the revised notebook. I did some corrections and got some equations. But I think there is

Could anyone please help me to find find functional derivative of some expression in using Xact

Hi, Oh I see, FreeIndsDown[] is not supposed to sort the indices. In that case, both functions are

Hi! This is a name conflict with new functions in Mathematica 14.3. José should soon release a new

Hi, I am trying to perturbed Einstein equation for Schwarzschild background metric, essentiality want

Hi, Thank you for the command suggestions! They work as needed and give the desired output. I think

Hi, I have made a small notebook which explains how to obtain perturbation equations in a synchronous

Hi I am not sure I understand what you want. Is your dX factor fixed? You have defined it as a 1-form