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Re: How to Extract a common factor from a Sum
*To*: mathgroup at smc.vnet.net
*Subject*: [mg12987] Re: How to Extract a common factor from a Sum
*From*: Tobias Oed <tobias at physics.odu.edu>
*Date*: Sun, 28 Jun 1998 02:52:10 -0400
*Organization*: Old Dominion University
*References*: <6mq9k4$2pi@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
Frank Loewenthal wrote:
> Hi Folks
>
> I have a symbolical sum like
>
> expr = Sum[ a f[k], {k,1,N}] and I want to force Mathematica to write
> this as
> expr = a Sum[f[k], {k,1,N}]
>
> What ever I tried it did not work.
> Does anyone knows a solution?
>
> Thanks in advance
>
> Frank
I made a little error in my previous posting:
In[1]:= Unprotect[Sum]
Out[1]= {Sum}
In[2]:= Sum[a_ b__,{k_,i__}] /; FreeQ[a,k] := a Sum[b,{k,i}]
In[3]:= Sum[a f[k] g[k],{k,1,N}]
Sum::itform: Argument g[k] at position 2
does not have the correct form for an iterator.
Out[3]= a Sum[f[k], g[k], {k, 1, N}]
you need to wrap the b in Times:
In[4]:= Sum[a_ b__,{k_,i__}] /; FreeQ[a,k] := a Sum[Times[b],{k,i}]
In[5]:= Sum[a f[k] g[k],{k,1,N}]
Out[5]= a Sum[f[k] g[k], {k, 1, N}]
Tobias
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