Re: Problem with using /.

*To*: mathgroup at smc.vnet.net*Subject*: [mg69246] Re: Problem with using /.*From*: "Nag" <Naga1010 at gmail.com>*Date*: Sun, 3 Sep 2006 23:46:33 -0400 (EDT)*References*: <edadil$pd0$1@smc.vnet.net><eddqil$3uh$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Peter Pein wrote: > > Hi Nag, > > you try to assign a value to Sum[...]; this becomes Set[Sum[...],0] -- > but Sum is a Protected function. Furthermore, Set is not Rule. > > I guess you wanted: > > M[j_, t_, k_] := FullSimplify[D[MGF[t, k], {t, j}] /. u -> 0, > Sum[Subscript[p, i], {i, 0, k}] == 1]; > > You can use Simplfy instead of FullSimplify too. > > Peter Hi Peter: The = was a typo. It should have been a -> With -> there is no error but the substitution doesn't happen. The substitution works when I use == as you suggested. Why does -> not work but == work? Thanks Nag