How to Extract a common factor from a Sum

*To*: mathgroup at smc.vnet.net*Subject*: [mg12882] How to Extract a common factor from a Sum*From*: loewenthal at iap.unibe.ch (Frank Loewenthal)*Date*: Wed, 24 Jun 1998 03:44:08 -0400*Sender*: owner-wri-mathgroup at wolfram.com

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