       Re: How to Extract a common factor from a Sum

• To: mathgroup at smc.vnet.net
• Subject: [mg12963] Re: How to Extract a common factor from a Sum
• From: colin at tri.org.au (Colin Rose)
• Date: Sun, 28 Jun 1998 02:51:52 -0400
• Organization: Theoretical Research Institute
• References: <6mq9k4\$2pi@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```loewenthal at iap.unibe.ch (Frank Loewenthal) wrote:

> 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?

MyRule := Sum[u_. w_,{k_,a_,b_}] :> u Sum[w,{k,a,b}] /;
FreeQ[u,k]==True

Example:

In[]   expr = Sum[a b c f[k], {k, 1, n}];

In[]   expr //. MyRule

Out[]  a b c Sum[f[k], {k, 1, n}]

Hope this helps

Cheerio

Colin

Colin Rose
tr(I)    -  Theoretical Research Institute
__________________________________________ colin at tri.org.au
http://www.tri.org.au/

```

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