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Re: How to Extract a common factor from a Sum


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



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