Re: How to Extract a common factor from a Sum

*To*: mathgroup at smc.vnet.net*Subject*: [mg12987] Re: How to Extract a common factor from a Sum*From*: Tobias Oed <tobias at physics.odu.edu>*Date*: Sun, 28 Jun 1998 02:52:10 -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 I made a little error in my previous posting: In[1]:= Unprotect[Sum] Out[1]= {Sum} In[2]:= Sum[a_ b__,{k_,i__}] /; FreeQ[a,k] := a Sum[b,{k,i}] In[3]:= Sum[a f[k] g[k],{k,1,N}] Sum::itform: Argument g[k] at position 2 does not have the correct form for an iterator. Out[3]= a Sum[f[k], g[k], {k, 1, N}] you need to wrap the b in Times: In[4]:= Sum[a_ b__,{k_,i__}] /; FreeQ[a,k] := a Sum[Times[b],{k,i}] In[5]:= Sum[a f[k] g[k],{k,1,N}] Out[5]= a Sum[f[k] g[k], {k, 1, N}] Tobias