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Re: Some questions about vector

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  • Subject: [mg74558] Re: [mg74543] Some questions about vector
  • From: bsyehuda at
  • Date: Mon, 26 Mar 2007 02:05:20 -0500 (EST)

Since N is not defined, the sum is not expanded, and therefor the pattern
v[i] is not found by the derivative operator and the result is zero.
I find it justified since you cannot differentiate by v[i] while N (better
to use another variable name in this case) is not defined and then  the
result can be zero (i is smaller then zero or greater then N) or "i"

anyway, having definite values for N and i return the desired result
On 3/25/07, Qi Zhang <qzhang81 at> 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.
> --
> Best,
> Qi Zhang

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