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MathGroup Archive 2009

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Taking the derivative of a summation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg95820] Taking the derivative of a summation
  • From: Alberto Lioy <nonalignmentpact at yahoo.it>
  • Date: Wed, 28 Jan 2009 06:29:26 -0500 (EST)

Hi everybody, I am an italian student in economics who is trying to solve an utility maximization problem (in Blanchard-Kiyotaki model)...I'm actually in trouble because I cannot have mathematica 5 take the derivative of a symbolic summation...

the case is the one of a summation of c(i)*p(i) with i between 1 and k
I want to take the derivative for a generic element c(i)  (p(i) is taken as fixed)

.now if I do D[Sum[Subscript[C,i]*Subscript[P, i], {i, 1, k}, Subscript[c, i]]
the program answers with Sum[Subscript[P, i], {i, 1, k}]
which is the summation of the p(i) elements...

I think that mathematica did not understand that I wanted to take the derivative for A generic element and gave me instead the derivative for EACH element..
I tried to take the dervative of c(1), c(k) or whatever...and it answers 0 (as was expectable)

the result I'm trying to obtain is p(i)...

maybe I just don't use the right commands...could anybody give me a piece of advice? thank you in any case


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