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