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?
>
>
> 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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