[GiNaC-list] Differentiation of a function with respect to a tensor

[Bernardo Rocha]

Dear Stephen,

thanks a lot for your support. I was considering if doing it this way it
would work, now I got my confirmation. Thanks a lot again.

Best regards,
Bernardo


2011/2/11 Stephen Montgomery-Smith <stephen at missouri.edu>

> Bernardo Rocha wrote:
>
>> Hi everyone,
>>
>> I've recently discovered GiNaC and I'm really excited about its
>> capabilities. There is one thing that I would like to know if it is able
>> to do that I haven't found in the tutorial.pdf or in any other place
>> that I've searched.
>>
>> I would like to know if, given a function \Psi=\Psi(E), like the strain
>> energy function for the St. Venant-Kirchhoff material
>>
>> \Psi(E) = 0.5 * \lambda * (tr E)^2 + \mu E:E
>>
>> is it possible to differentiate it with respect to E, that is i would
>> like to compute \frac{\partial \Psi}{\partial E}. If this is possible,
>> could someone please send some examples or maybe point to which classes
>> should I use to do that?
>>
>> That's all for now. Many thanks in advance.
>>
>> Best regards,
>> Bernardo M. R.
>>
>>
>>
> Couldn't you do it this way?  Write \Psi(E) as an expression involving the
> variables e11,e12,e13,...,e33 which are the entries of E.  Then compute the
> partial derivatives \frac{\partial \Psi}{\partial eij} for 1<=i,j<=3.
>  (Presumably you suppose that E is symmetric so only six partial derivatives
> need to be computed, but even if it is not necessarily symmetric you still
> only need 9 partial derivatives.)  Just store this as something like:
>
> expr dPsi_dE[3][3]
>
> or
>
> vector<expr> dPsi_dE
>
> or something similar.
>
>
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