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.