       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 /.
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 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.

```

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