Services & Resources / Wolfram Forums
MathGroup Archive
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2007

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Some questions about vector

  • To: mathgroup at
  • 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

  • Prev by Date: Re: Which Mathematica product should I get?
  • Next by Date: Re: No O Reilly Mathematica Cookbook
  • Previous by thread: Some questions about vector
  • Next by thread: Re: Some questions about vector