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Re: Some questions about vector

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  • Subject: [mg74567] Re: Some questions about vector
  • From: dh <dh at>
  • Date: Tue, 27 Mar 2007 03:52:14 -0500 (EST)
  • References: <eu54qn$l30$>

Hi Qi,

Mathematica treats your command mechanical. It takes the derivative of 

Sum[i v[i],{i,1,N}] treating v[i] as a variable. Well this gives 

Sum[i,{i,1,N}] and this evaluates to what you got.

Try also D[f[v],v[j]] and you get zero! As far as I know, it is not 

possible to take the derivative with respect to a not yet determined 

variable y[j]. To be on the save side, in D[..,x], x should be an atomic 



Qi Zhang wrote:

> input:


> Clear[f];

> f[v] = Sum[(i*v[i]), {i, 1, N}]

> D[f[v], v[i]]


> the output is


> \!\(1\/2\ N\ \((1 + N)\)\)


> However what I expect is i


> What should I do in order to get the the answer I expected?


> Thanks a lot.


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