# [Maxima] Differentiation in sum

Alexey Beshenov al at beshenov.ru
Sun Jan 11 06:07:03 CST 2009

On Saturday 10 January 2009 00:32:06 Mykhailo Korbakov wrote:
> I'm very new to Maxima and symbolic computations at all, so please,
> don't make laugh of me if my question is trivial or stupid :)
>
> I have a simple polynomial function y:tpsum(i=0, N, w_i*x^i)
> Is it possible to differentiate by w_1, w_2, w_j symbols? I'm
> getting '0' for result of diff(y, w_1). That's definitely not the
> 'x' I was hoping to obtain.

1. What is "tpsum"? I think Maxima doesn't know such function.

2. "w_1" is not "w_i at i=1". Maxima uses brackets for subscripted
names.

3. If you want to differentiate \sum_{i=0}^N w_i x^i, then the correct
expression would be diff(sum(w[i]*x^i,i,0,N), w[1]), but Maxima will
return 0 unless you fix N.

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Alexey Beshenov  http://beshenov.ru/