Re: How to Extract a common factor from a Sum

*To*: mathgroup at smc.vnet.net*Subject*: [mg13137] Re: How to Extract a common factor from a Sum*From*: "Allan Hayes" <hay at haystack.demon.cc.uk>*Date*: Mon, 13 Jul 1998 07:41:51 -0400*References*: <6mq9k4$2pi@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Frank Loewenthal wrote in message <6mq9k4$2pi at smc.vnet.net>... >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 > Another variant: factorOut = Sum[expr_ a_,its__]/; FreeQ[a, Alternatives@@First/@{its}]:>a Sum[expr, its] Test Sum[ a b f[k,l]g[k] z, {k,1,N},{l,1,M}]/.factorOut a b z Sum[f[k, l] g[k], {k, 1, N}, {l, 1, M}] ------------------------------------------------------------- Allan Hayes Training and Consulting Leicester UK http://www.haystack.demon.co.uk hay at haystack.demon.co.uk voice: +44 (0)116 271 4198 fax: +44(0)116 271 8642