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Re: How to Extract a common factor from a Sum
*To*: mathgroup at smc.vnet.net
*Subject*: [mg12950] Re: How to Extract a common factor from a Sum
*From*: "Stephen P Luttrell" <luttrell at signal.dra.hmg.gb>
*Date*: Sun, 28 Jun 1998 02:51:43 -0400
*Organization*: Defence Evaluation and Research Agency
*References*: <6mq9k4$2pi@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
>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?
Add the following definition to Sum:
Unprotect[Sum];
Sum[a x_,y__]:=a Sum[x,y];
Protect[Sum];
where the double blank __ allows multiple sums to be matched.
Now evaluate your expression
Sum[ a f[k], {k,1,N}]
to obtain the result
a Sum[f[k], {k, 1, N}]
Here is a double summation
Sum[ a f[k,l], {k,1,N},{l,1,M}]
which yields the output
a Sum[f[k,l],{k,1,N},{l,1,M}]
===============================================================
Stephen P Luttrell
luttrell at signal.dera.gov.uk
Adaptive Systems Theory 01684-894046 (phone)
Room EX21, DERA 01684-894384 (fax)
Malvern, Worcs, WR14 3PS, U.K.
http://www.dera.gov.uk/cis5pip/Welcome.html
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