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? > > 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