Re: How to Extract a common factor from a Sum
- To: mathgroup at smc.vnet.net
- Subject: [mg12986] Re: How to Extract a common factor from a Sum
- From: Tobias Oed <tobias at physics.odu.edu>
- Date: Sun, 28 Jun 1998 02:52:09 -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
How about
In[1]:= Unprotect[Sum]
Out[1]= {Sum}
In[2]:= Sum[a_ b__,{k_,i__}] /; FreeQ[a,k] := a Sum[b,{k,i}]
In[3]:= Protect[Sum]
Out[3]= {Sum}
In[4]:= Sum[ a f[k], {k,1,N}]
Out[4]= a Sum[f[k], {k, 1, N}]
Tobias