Re: A Question about Expression Simplication

*To*: mathgroup at smc.vnet.net*Subject*: [mg31572] Re: A Question about Expression Simplication*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Wed, 14 Nov 2001 03:41:48 -0500 (EST)*Organization*: Universitaet Leipzig*References*: <9sl2mo$dhu$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hi, it does not work at all, because Sum[] has the attribute HoldAll and this will prevent almost all simplifications. The reason is, that Sum[a+b,{i,1,Infinity}] may be convergent but Sum[a,{i,1,Infinity}+Sum[b,{i,1,Infinity}] may be undetermined. You notation Sum[x_i, {i,1,2,T}] would mean run i from 1 to 2 in steps of T. You must use rules explicit to handle Sum[x_i, {i,1,T}] - x_2 and Sum[x[i], {i, 1, T}] - x[2] /. Sum[a_, {i_, 1, n_}] :> Sum[a, {i, 2}] + Sum[a, {i, 3, T}] works as it should but will be an error for T<3 Regards Jens Lewis wrote: > > Hi MathGroup, > > I am a beginner to Mathematica. Please help me solve the following problem. > > I am trying to manipulate expressions like: > > Sum[x_i, {i,1,T}] - x_2 or Sum[x_i, {i,1,T}] - Sum[x_i, {i,1,2,T}] > > I want to keep the upper limit as T, not any specific numeric value. I > tried to use Simplify as in > > Simplify[Sum[x_i, {i,1,T}] - x_2] > > But the x_2 term doesn't get cancelled out. I also tried: > > Simplify[Sum[x_i, {i,1,T}] - x_2, T>2], > > but it didn't seem to work either. So I am wondering if the simplication > only works when I specify the value for T. Any advice, please? > > Thank you very much! > > Best, > Lewis.