IndexCollect

[olse barn]

Hey everyone, 

I have a pretty long expression which looks something like this:

... + PD[-i][PD[-j][f[]]] * somethingelse1 +... + PD[-k][PD[-l][f[]]] * somethingelse2 + ... .

I would now like to collect only those terms that are multiplied by 

PD[-index1][PD[-index2][f[]]]

where index1 and index2 can be any element of the set of indices that I defined for the manifold.

Is there a way to do this? I.e. can I collect partial derivative terms with "undefined" indices and not having to Collect each combination "by hand"?

All the best.

[olse barn]

Hey again, I already solved it by multiplying every f[] with a constant defined by DefConstantSymbol[coef].

[Thomas Bäckdahl]

Hi!

Yes, multiplying with constants and collecting constants is a useful method. The constant can be replaced with 1 after the collection too.

Also note that Collect works with patterns too, so you can do something like

Collect[%, {PD[_][PD[_][f[]]]}]

However, this will not bundle together expressions like this with different indices.
The function IndexCollect tries to handle that kind of problem though.

Regards
Thomas

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[olse barn]

thank you for your alternative proposal!