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RE: Sum expansion

• To: mathgroup at smc.vnet.net
• Subject: [mg32668] RE: [mg32630] Sum expansion
• From: "Wolf, Hartmut" <Hartmut.Wolf at t-systems.com>
• Date: Mon, 4 Feb 2002 03:23:36 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

> -----Original Message-----
> From: Cyril Fischer [mailto:fischerc at itam.cas.cz]
To: mathgroup at smc.vnet.net
> Sent: Friday, February 01, 2002 8:03 AM
> To: mathgroup at smc.vnet.net
> Subject: [mg32668] [mg32630] Sum expansion
> 
> 
> 
> I would like to arrange exression
> A Sum [ B_i ,{i,1,s}]
> into expression
> Sum [ A B_i, {i,1,s} ]
> 
> Following transformation code gives unwanted answer:
> A*Sum[Subscript[B, i], {i, 1, s}] /.
>   a_*Sum[b_, {i, 1, s}] -> Sum[a*b, {i, 1, s}]
> Result is
> A s Bi
> 
> I understand, why it behawes this way, but I dont know the 
> correct way.
> 
> Thanks for your hints,
> Cyril
> 
> 
Cyril,

use RuleDelayed

In[9]:= A*Sum[Subscript[B, i], {i, 1, s}] /. 
  a_*Sum[b_, {i, 1, s}] :> Sum[a*b, {i, 1, s}]
Out[9]= 
Sum[A*Subscript[B, i], {i, 1, s}]

otherwise the rhs of Rule can be evaluated (prematurely for you). 

[This would not have happend, if a dependence on i had been preserved:

In[10]:= A*Sum[Subscript[B, i], {i, 1, s}] /. 
  a_*Sum[Subscript[b_, i], {i, 1, s}] -> 
       Sum[a*Subscript[b, i], {i, 1, s}]
Out[10]=
Sum[A*Subscript[B, i], {i, 1, s}]

...]

This is enough in this case. Otherwise, prevent evaluation of the lhs with
Unevaluated, of the pattern with HoldPattern (and use RuleDelayed of course)

--
Hartmut