Questions tagged [xact]
Questions about xAct, a suite of packages for tensor computer algebra, which includes the packages xCore, xPerm, xTensor, and xCoba.
88
questions
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views
xAct: How to identify and split module dummy indices?
Hi I'm using the package xAct but I'm running into an issue. It seems that before using //ChristoffelToGradMetric the christoffel connection has indices (which can ...
- 1
0
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0
answers
61
views
Counting multiple $-sign string indices in output and replacing them by normal indices
I'm using the package xTensor in xAct. My issue is that inside a module indices are replaced by strings of the form $##### which make the final output unreadable. In a related post Print expressions ...
- 1
1
vote
0
answers
23
views
Commutation of the partial derivatives on scalar in xAct
I do not know how to automatically commute partial derivatives on a scalar and simplify it. For example, if I have:
$$\partial_a \partial_b \partial_c f- \partial_c \partial_a \partial_b f $$ where $f$...
- 57
2
votes
1
answer
46
views
Symbolic representation of large output
The package xAct does something really nice when it handles large outputs, which is to represent the expression as a symbol and display that symbol when the cursor hovers over it. For example,
...
- 413
1
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0
answers
42
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How to use VarD to vary $\partial T$
I know that how the VarD works which is $ VarD[T, covd][expr] $. What about when I want to vary to a partial of a tensor ($\partial T)$? Let me say a simple question I wanted to test:
$ L = \partial_{...
- 57
2
votes
1
answer
59
views
in xAct, how to set the components of a tensor to be zero
OBASuppose there is a vector $A^{a}$ appears in the result in different component form like $A^{\mu},A^{\nu},A^{\alpha}...$,
I want all of them to be zero because the components of the tensor are zero,...
- 33
3
votes
1
answer
86
views
how to split indices for a Spinor object?
I am wondering if anybody can help me with my question. My main relation is from gravity and is known as self-dual Riemann. When it is written in the Spinorial form it is like:
$$G_{\mu \nu \alpha\...
- 57
1
vote
1
answer
38
views
xAct : how to define f to be a function of two variables?
f is a function of the coordinates x and y, how can I use the command DefScalarFunction to do this?
...
- 33
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votes
0
answers
30
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calculate the component of a tensor-like quantity using xCoba,and how to use ContactBasis ,TraceBasisDummy
I am trying to do perturbation, it involved terms like this:
$\partial_{l}(g^{lp} g^{kq} h_{pq}) (\partial_{i}g_{jk})
$
I want to obtain the simplified result of the components {i,j}
all other indices ...
- 33
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votes
0
answers
16
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"Uncaught Throw[Null]","VBundleOfMetric" and "MetricsOfVBundle" when using package "xAct"
The codes are below:
Remove["Global`*"]
Then:
...
2
votes
0
answers
55
views
Manipulating tensorial terms with xAct, ToCanonical is not perfoming well
I am trying to calculate perturbations of some complicated Lagrangians. Using xAct and more specifically VarD to perform the perturbations the expressions become ...
- 21
2
votes
0
answers
52
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Not able to impose rules on xPert perturbations
Whenever I try to define rules to xPert perturbations, they seem not to work at all. For instance, I am interested in considering traceless and divergence-free perturbations and defining
...
1
vote
0
answers
68
views
xAct tensor product
I am a beginner of xAct for computation in GR. I am doing component computation in specified charts, and quite often I need to extract the list of components by the function ...
- 11
2
votes
2
answers
225
views
xAct xCoba: How to extract components of CTensors correctly
I'm working in a specific chart ch with a metric and associated covariant derivative cd:
...
- 23
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votes
0
answers
49
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amount of Riemann scalar on a spherical shell in xAct
how can I Calculate the amount of Riemann scalar on a spherical shell?
(i used xCoba and I defined a manifold, then I defined a metric and a chart with theta and phi coordinates but I don't know how ...
0
votes
0
answers
52
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velocity four-vector definition by xAct
I want to define the velocity vector by the following relation and evaluate the norm of the vector by using the Minkowski metric.
I have defined the tensor x (position tensor) in xAct, but I don't ...
2
votes
1
answer
82
views
Calculating the Laplacian operator using xCoba
I would like to attempt to calculate the laplacian operator using index notation and shoow that it gives the usually expected laplacian in spherical coordinates. I am able to do most of the steps ...
- 635
2
votes
1
answer
124
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Dealing with covariant derivatives (xCoba)
I do not understand why the output of the two commands:
...
2
votes
2
answers
106
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Taking partial derivatives of a scalar function with a defined basis and chart in xCoba/xAct
I want to work in the Schwarzchild metric in 2 dimensions with the coordinates $v, r, \theta$. My hope was that I could define a scalar function made up of 2 other scalar functions:
$$\Psi(v,r,\theta) ...
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1
vote
0
answers
59
views
Perturbation of Einstein tensor around a fixed background in Xact [duplicate]
I want to compute Ricci tensor expansion around a fixed curved background. It is easy to find the formal expansion around the background metric, but I have a reference metric with known components and ...
- 149
0
votes
0
answers
98
views
Metric perturbation calculations in xAct
Any hints to calculating the following two-point function, which is not time-ordered, in xAct?
$$
\begin{equation}
<h_{\mu\nu}(x) h_{\alpha\beta}(y)>
\end{equation}
$$
...
- 1
1
vote
0
answers
56
views
Use xTensor to do the Ricci decomposition
I am new to xTensor. I would like to perform the Ricci decomposition with xTensor, but I do not have any clue. I want to replace the full expression $R_{i,j,k,l}=S_{i,j,k,l}+E_{i,j,k,l}+W_{i,j,k,l}$ ...
2
votes
0
answers
58
views
Covariant divergence in xAct
I'm using xAct for calculation of expansion scalar for Kerr in Boyer-Linquist coordinates. Expansion scalar is given as a covariant divergence:
with
where \Delta and \Sigma are defined as
In the ...
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3
votes
1
answer
76
views
Neglecting higher order derivatives terms in xAct
For some gravity theory coupled to a scalar field $\phi$, I obtain, in xAct, the equations of motion
$$\mathcal{E}_{ab}=R_{ab} (\nabla_{c}\phi \nabla^{c}\phi) + 2 R \nabla_{a}\phi\nabla_{b}\phi + 6 \...
4
votes
1
answer
144
views
Evaluating covariant derivative terms of a scalar function (xAct, xTras)
I am trying to use xAct package to calculate certain mixing terms of $\nabla_{\chi} b$ of a scalar function $b[ \tau, r, \theta, \phi]$ for a modified metric. Here is my Code:
...
1
vote
0
answers
92
views
Basis object appearing
I want to calculate explicitly an expression involving covariant derivatives of a Riemann tensor. However, in the output I get Basis with indices generated, which I am not sure how to get rid off and ...
- 111
1
vote
0
answers
54
views
Problem initializing xTensor package
I am using Mathematica 12.2 on Windows 10 and I am using the xTensor package for my tensor calculations.
However, since today, I cannot run the xTensor package. I get the following error. Can anybody ...
- 111
1
vote
1
answer
167
views
Converting tensorial expressions in xAcT to LaTeX
I'm trying to convert tensorial expressions from xAct to LaTeX format using the TeXForm command. This seems to work fine when using covariant derivatives (As in Copy xAct expression to LaTeX). However,...
0
votes
0
answers
74
views
How To Define An Outer Product (in xAct)?
I am working with xAct / xCoba / xTras and I would like to construct a matrix out of a vector.
So, I have a vector V on a manifold ...
- 151
2
votes
1
answer
144
views
Contracting a Tensor with a CTensor in xAct
I'm trying to contract two tensors that I made in xAct to get a simple result. First I set up a Cartesian space
...
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1
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0
answers
166
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How to define a divergence-free vector or a traceless tensor in xAct?
I'm trying to implement a Friedmann-Lemaître-Robertson-Walker metric + a perturbation in a scalar-vector-tensor split in mathematica, using xAct. The S-V-T decomposition comes with some rules for the ...
- 151
1
vote
0
answers
204
views
Equation of Motion: Perturb Scalar Field and flat FLRW Metric (xAct)
Is there a way to get the equations of motion (eom) of a scalar field from a Lagrangian in a perturbed FLRW metric? I want to perturb the field as well as the metric to 1st order. What I did so far:
...
- 151
1
vote
0
answers
76
views
Product of manifolds with non−zero non−diagonal boxes in the metric
I'm trying to construct in xAct a metric like this
where $g_{\mu\nu}$ is 4-dimentional, and $g_{MN}$ - 5-dimentional, $A_\mu$ - 4-vector and $\phi$ is a scalar field.
I already tried to do it like ...
- 11
2
votes
0
answers
39
views
Mathematica parallelization error
I am trying to parallelize some code in Mathematica 11.3 under Ubuntu 20.04, but I get the following errors:
the code is a s follows:
...
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18
votes
3
answers
441
views
Coordinate-free derivative
Given the function
\begin{align*}
f \colon \mathbb{R}^n &\to \mathbb{R}^n\\
v&\mapsto \dfrac{v}{\|v\|},
\end{align*}
I would like to compute the derivative of $f$, that is $df(v)$. It is ...
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1
vote
1
answer
201
views
How to associate a tetrad as a basis of a metric mathematica
It's my first time here, and it's the first time that I try to work with Mathematica, so I am a bit lost! If somebody could help me I'll always be grateful!
I need to define a generale basis for a ...
2
votes
0
answers
152
views
How to define tensors similar to Riemann tensor in xAct xTensor xTras
I'm working on a project where we deal with objects very similar to Riemann tensor. In fact they are Christoffel symbols for higher spins. I'm using xTras for modeling problems in wolfram mathematica. ...
- 135
1
vote
0
answers
96
views
How can I define a metric with (+,-,-,-) signature in xTensor
Can I define the metric with (+,-,-,-) sign by using the DefMetric[1,metric[-a,-b], CD]? Does "1" in the first place refer to the sign of the time component of the metric or the determinent. If it ...
2
votes
1
answer
116
views
2
votes
1
answer
355
views
xAct working with an unspecified function
I am trying to use xCoba with a diagonal metric depending on an unspecified function f[r], but I get the errors:
CTensor::...
- 6,104
1
vote
1
answer
791
views
Defined a "mixed metric" in Xact/Xcoba
I would like to be able to define a metric of this form in XAct, :
$$G_{AB}dX^A dX^B = g_{\mu\nu}(x^\mu,y^i)dx^\mu dx^\nu+h_{ij}(y^i)dy^i dy^j$$
So that I can have the general form of the different ...
- 123
2
votes
0
answers
261
views
Contraction of tensors in chart (xCoba)
I have to do some calculations on modified gravity in xAct. The question is how to construct tensors, made of other tensors with contraction in particular chart. Without chart it can be done with <...
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2
votes
1
answer
417
views
Defining a tensor from an expression of other tensors in xAct
There's been a number of people asking how to set a tensor-valued variable in xAct; equivalently, how to define a new tensor from an expression of existing tensors.
For example, a very simple ...
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1
vote
1
answer
162
views
Is it possible to defining a scalar with CTensor?
I have a tensor (rank zero) or a scalar $\phi$ which defined as
DefTensor[\[Phi][], M]
on manifold $M$.
After defining the metric, how can I use ...
- 653
3
votes
1
answer
584
views
How to compute functional derivative against vector in xAct, xTensor or xTras?
I want to compute the functional derivative against vectors.
For example, I have an object that looks like this
$R = h_{ijkl}a^i a^j a^k a^l$
I need to compute $\frac{\delta R}{\delta a^p}= 4 h_{...
- 135
2
votes
1
answer
371
views
Kretschmann scalar in xTensor and xCoba
How do I calculate the Kretschmann scalar $K=R_{abcd}R^{abcd}$ with xTensor and xCoba? I have found the functions Kretschmann and ...
- 6,104
2
votes
1
answer
515
views
How to work with Levi civita tensor in xAxt?
Is there any command to work with the Levici vita tensor? In fact, I found LeviCivitaTensor, however, in calculations it doesn't seem practical.
see the example:
<...
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1
vote
0
answers
50
views
Derivatives with dummy indices not commutative after 3d derivative?
Recently I've encountered the following issue in xAct.
I've defined the Manifold and tensors in the following manner
Then consider the following expressions.
In the case where there are 2 ...
- 135
1
vote
0
answers
66
views
How to combine symmetrized tensor expressions into single expression in xAct
Let's sayI have the following expression in xAct
where covariant derivatives are commutative, $\epsilon$ and $h$ are symmetric tensors.
As we can see this expression is equivalent to
If we ...
- 135
2
votes
0
answers
125
views
How to remove the Riemann tensor derivatives using Bianchi identity?
For the third-order Lovelock gravity, after varying the Lagrangian versus the metric, I found some derivatives of the Riemann tensor which should not be appeared. How can I remove them? Maybe using ...
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