## 10699 Reputation

16 years, 199 days

## Help pages for Physics and and Different...

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

## @subzero  Your question is too spe...

Your question is too specific to answer without seeing details. First of all, do you have Maple? If so, open please the help page ?Physics:-Christoffel. There is a defining formula there. The starting point is for you to tell, in formulas, on a Maple worksheet, exactly what (apparently different?) definition you want to use. Without that I can't help you more than the generic comments in the previous two replies, or saying my opinion, that Maple has more thorough and flexible tools for working with general relativity than any other platform (as in: for GR, if not in Maple, then nowhere).

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

## UPDATE Mar/21: This is now ...

UPDATE Mar/21: This is now fixed and the fix distributed for everybody using Maple 2021 within the Maplesoft Physics Updates v.934 and newer.

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

## UPDATE Mar/21: This is now fixed an...

UPDATE Mar/21: This is now fixed and the fix distributed for everybody using Maple 2021 within the Maplesoft Physics Updates v.934 and newer.

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

## @subzero You can't do it the wa...

You can't do it the way you say, the same way you can note redefine the exponential function and expect that everything else uses your new definition. Christoffel has a unique definition. That definition is used in several places. If you post your problem or an example that illustrates what you need more specifically, then one could imagine a solution.

Meantime, what you can do is ignore entirely that all these tensors exist predefined according to textbook definitions (Christoffel, Ricci and Riemann) and define yours, say C, R, Ri, where your C I imagine is in terms of the metric g and its derivatives, R is some contraction of Ri and Ri is a function of C, g and its derivatives. To define a W traceless tensor related to Ri is also trivial. All that can be done defining tensors as explained in the Physics,Tensors help page, the sections on how to define tensors. Actually, it wouldn't take more than 5 minutes to define three or four tensors like those.

To summarize, you can always ignore the existing definition of these tensors and work with an entire set of other ones that you define with ease in a few minutes at most. It is simpler if you could show, on a Maple worksheet, more specifically, what you need and how do you intend to use it. There may be other more convenient solutions.

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft.

## @nm Why don't you write an emai...

Copy As LaTeX works pretty-well, and is a very nice feature of Maple 2021. Don't give up that. Why don't you write an email to support@maplesoft.com telling the problem? I'm sure they will do their best to help you diagnose and fix the problem, which, by the way, is not related to Physics, the Updates or anything else. I would try to uninstall Maple and then use a File Manager to wipe out every remaining for-sure. Then re-install; though I suppose you already tried that, anyway, support is the way to go, I think.

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft.

## @nm  It is difficult to imagine th...

It is difficult to imagine the problem; guessing here, could it be that you have another library interfering? Maybe some Maple 2020 library around in a personal directory ... or the Physics Updates installed by hand?

Take a look at the directories mentioned in the variable libname and make sure you don't have any additional libraries around. Then, Copy As LaTeX should work right away, even if you don't have the Maplesoft Physics Updates installed. And if you have it, as said below, I implemented a new option so that things get copied as output, not as input.

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

## @mthkvv  I looked; good feedback y...

I looked; good feedback yours. Some improvements are needed here. I am busy at this moment. I will try to address them the next week, partially or entirely, and write here again.

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

## @mthkvv  Yes. I think you can get (...

@mthkvv
Yes. I think you can get (96.8) directly from the definition of t you do with the so small-expression (96.5). So isolate t[~i, ~k] in (96.5), then convert(%, Christoffel). You may need to collect g_, factor, or the like to get it written so organized, but you are expected to get (96.8). Take a look at the Sec II, subsection 20 of ?Physics,Tensors to familiarize yourself with the idea; the conversion network is vast and extremely useful for derivations like the ones you are pointing at in Landau's book.

Caveats: in the above, I'm saying, but didn't actually do the computation - if you do the computation and find something not working as I am saying, please let me know; in that case, I will take a closer look.

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft.

## @mthkvv  Yes, you can. Define ...

Yes, you can. Define the tensors, lambda and h (92.[2,3]), then T is the EnergyMomentum (or use the Einstein's tensor); next, define `t` using (96.5), then input (96.8) and verify equality, for instance as done in the previous reply above. Once you verified you entered your tensor definitions without typographical mistakes, go with convert((96.8), g_), and you will receive (96.9).

Important: all these things are explained in ?Physics,Tensors, including the conversion network in Sec II, subsection 20; then you already know how to define the tensors (from your kerr.mw). Using inert tensors sometimes help with these demonstrations (there is always more than one way to do them), and any intermediate tensorial manipulation is also expected to be explained in ?Physics,Tensors. If it is not there, let me know, please, and I will include it.

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

## Yes....

What follows works fine with g_[arbitrary] but, for experimentation, try with something simpler, that allows you to verify almost by eye:

 (1)

To verify (86.9), define  the tensor

 (2)

Define now as antisymmetric; use the  option to ensure the antisymmetry is automatically used when computing with components, used when verifying (86.10). Use compact display for both:

 (3)

 (4)

This is Landau & Lifshitz (86.9)

 (5)

 (6)

 (7)

Check that the left and right hand sides of this equation are the equal

 (8)

There are some additional manipulation capabilities for the intermediate steps, taking advantage of the implementation of inert tensors (see Physics, Tensors, Section 1, subsection 7 ). For example, go one step only for the right-hand side of (5)

 (9)

Verify this expansion

 (10)

 (11)

Good. Now the same with (86.10)

 (12)

 (13)

As with (86.9), you also have some control over the steps:

 (14)

Here are three different ways to verify these outputs are correct, the left and right hand sides have the same value.

1) First a direct approach as before

 (15)

 (16)

 (17)

2) First Simplify (13) to take into account the antisymmetry of

 (18)

Turn the alpha index in  contravariant so that when you sum over the repeated indices you compare contravariant with contravariant components of F

 (19)

 (20)

 (21)

3) This is frequently the simpler and faster approach towards verification, just ask the computer to compare component by component. Here I take the left-hand side minus the right-hand side, and simplify

 (22)

When you have two indices, you see a Matrix of zeros. And what in the case where you have too many free indices? The above will be an Array where you cannot see all the components. OK, but if those components are all equal to zero, then ArrayElems  applied to that Array returns an empty set. So you can take advantage of that - an easy way to see all the components are zero. Try with (22) to see that in action

 (23)

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

## @mykola  The main problem with uns...

The main problem with unspecified dimensions is that you cannot compute the components of the metric, and so you cannot sum over repeated indices nor compute the determinant or the trace; and with that you lack support for operations with all the other tensors: you basically cannot compute any of them, at the core you miss an explicit expression for the Christoffel symbols. All that, together, restricts significantly the computations you can perform.

That said, some things could be done, e.g. simplification using Einstein's sum rule and differentiation. If you could perhaps formulate on a Maple sheet a computation where an unspecified dimension is relevant, what is the starting point and what is the result you wanted to derive from it. What the steps, Maybe an option and support for unspecified dimension can be implemented.

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

## Mapleprimes post on Wirtinger Derivative...

Well, not you, but I missed something. Wirtinger derivatives were implemented in Maple 18 years ago. You can see the routines: enter kernelopts(opaquemodules = false) followed by print(Physics:-DifferentiateComplexComponent) and display any print/foo foo is a complex component.

But the Maple18 implementation remained somewhat incomplete - differentiation w.r.t z worked well on expressions involving z and conjugate(z), but not differentiation w.r.t conjugate(z) when there is z around (your examples called my attention to that, thanks!). And I completely forgot about that, mainly because I always differentiate w.r.t z, not conjugate(z).

Anyway, the topic overall is relevant, I think. So besides completing the few missing steps since Maple 18, I prepared a Mapleprimes post about Wirtinger Derivatives. As usual, I don't discard there is more work to do, but up to what I could see on a Sunday morning :), the implementation is working as expected; and you, the more experts, can tell.

Best

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

## @mthkvv  :) In turn, I was impress...

:) In turn, I was impressed with the extensive use you do in your document kerr.mw of the ability to define tensors using tensorial expressions, including covariant derivatives and the determinant of the metric. By the way, maybe you are aware, maybe not, it is possible to work with the inert form of the determinant of the metric, including differentiation, instead of the computed form you use. Check the help page ?Physics,Tensors, Section II subsection 22.

PS: not everybody is aware, "distributed in the Maplesoft Physics Updates" means the fix is already present in the version of Maple 2022 under development.

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

## There is an issue in int, triggered...

There is an issue in int, triggered after assumptions are (automatically) placed on the coordinates t, r, theta, phi when you set them. I'm taking a look. A fix will be there in the next version of the Maplesoft Physics Updates for Maple 2021. By the way, the first version of the Physics Updates for Maple 2021 was posted earlier today. To install versions, you need to "install the package" first  (it is not sufficient to have it installed in Maple 2020). Also, the webpage still says "2020" but v.927 is for "2021".

Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft

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