       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?
>
>
> Frank

In:= Unprotect[Sum]

Out= {Sum}

In:= Sum[a_ b__,{k_,i__}] /; FreeQ[a,k] := a Sum[b,{k,i}]

In:= Protect[Sum]

Out= {Sum}

In:= Sum[ a f[k], {k,1,N}]

Out= a Sum[f[k], {k, 1, N}]

Tobias

```

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