Re: Some questions about vector

*To*: mathgroup at smc.vnet.net*Subject*: [mg74567] Re: Some questions about vector*From*: dh <dh at metrohm.ch>*Date*: Tue, 27 Mar 2007 03:52:14 -0500 (EST)*References*: <eu54qn$l30$1@smc.vnet.net>

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 symbol. Daniel 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. >