# [Maxima] Differentiate into a sum

sen1 at math.msu.edu sen1 at math.msu.edu
Mon Jul 2 12:16:42 CDT 2007

```Are the expressions
x[k]
independent?

You say "some function" of the sum.

Is this just the composition

g(u)

where
u = sum(a[k]*x[k])

where   a   and    x    are just vectors?

In that case,  du/d(x[k]) is just a[k], so perhaps

a[k]* subst(u= sum(a[k]*x[k],k,1,n), diff(g(u),u))

does what you want.

-sen

On Sun, 1 Jul 2007, Jordi Gutierrez Hermoso wrote:

> How can I do something like finding the derivative wrt to x[r] of some
> function of the sum sum(a[k]*x[k], k, 1, n)? With n an integer greater
> than 1 and r another integer between 1 and n, of course. I'd like
> Maxima to give a pretty answer whenever possible and not expand the
> summation.
>
> Thanks,
> - Jordi G. H.
> _______________________________________________
> Maxima mailing list
> Maxima at math.utexas.edu
> http://www.math.utexas.edu/mailman/listinfo/maxima
>

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