xAct Tensor Computer Algebra

Hi everyone, Can you help me? I'm working with a metric: DefManifold[M, dd, IndexRange[a, s]]

Hello, I am trying to write a simple notebook to compute the Kubo formula in an isotropic background.

Dear Thomas, Now it works! Thank you so much for your help! It turns out that perhaps either I forgot

The last time I used xAct in Mathematica 14.3 was at the beginning of November. Now, I had to check

Dear Héloïse, I have checked your different approaches. In particular, I am more familiar with the

Dear Asghar, I am not sure what you are referring to when you mention the standard textbook. I have

Dear I hope this message finds you well. I have attached a Mathematica notebook containing the

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