Parenthesis in expressions do not go away after Expand

[BN]

Hello everyone,

I have a strange thing happening - the expression seems unable to be rid of parentheses in (  )  around a single term. What I have is just taking the example from xAct to find EOM, with an additional command to perturb the EOM themselves. I have attached the notebook with the example. Note the terms in parentheses (...) when I continue to manipulate with the expression, they simply won't go away and I have no idea why. It doesn't care about Expand, as you can see... 

These terms are also in the xAct example nb itself, in the case of the scalar field.

So how do you get rid of this? The main problem is when I try to use Coba to trace over basis dummy and expand in basis, the terms with parentheses are always covariant derivatives of tensors and they do not get expanded at all. Am I missing something?

Best,

Branislav

[Thomas Bäckdahl]

Hi Branislav,

The parentheses are not important, they are just there in the output to make it clear that the derivative is not acting on any other factor. Try for instance cd[b]@h[LI[1], -a, -b] cd[e]@Riccicd[-c, -e]. Without the parentheses in the output form it might be interpreted in the wrong way by the user.

Hence, your problem must be with the way you are tracing basis dummies. In this case you must make sure that the covariant derivatives are first converted into partial derivatives with respect to your basis. Then you have to make sure that all your indices are converted into basis indices. Unfortunately, this might take applications of ToBasis.

For instance set d=2 in DefManifold and try
DefBasis[B2, TangentM, Range@2]
ToBasis[B2]@ToBasis[B2]@ChangeCovD[EOMGR1, cd, PDB2]
TraceBasisDummy@%

Then I think your example will work.

Regards
Thomas

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[BN]

Hi Thomas,

Thank you for your answer! Yes I see now, I just need to keep playing around with all the commands in the right order eventually and it works. Thank you.

Best regards,

Branislav