Re: transforming exponential of sums into product of exponentials

*To*: mathgroup at smc.vnet.net*Subject*: [mg48692] Re: transforming exponential of sums into product of exponentials*From*: "Steve Luttrell" <steve_usenet at _removemefirst_luttrell.org.uk>*Date*: Thu, 10 Jun 2004 02:44:30 -0400 (EDT)*References*: <ca6igm$j2s$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

The reason for this behaviour is that your rules do indeed get applied to give the result Exp[a]Exp[b], but then Mathematica "helpfully" simplifies this back to Exp[x+y] to give the result you see displayed. I generally find the easiest way around this sort of problem is to "fool" Mathematica so that it doesn't recognise the expression that you want to be output as one that it can "helpfully" simplify for you. You might try something like Exp[x + y] /. Exp[a_ + b_] -> exp[a]exp[b] which gives the output exp[x] exp[y] because Mathematica doesn't recognise the function exp[x] as one that it knows anything about, and indeed it is an entirely new function that you have decided to use. To make Mathematica do anything with exp you would need to define what is allowed. For instance making the following definition exp /: exp[x_]exp[y_] := exp[x + y] causes Mathematica to give the following output when evaluating Exp[x + y] /. Exp[a_ + b_] -> exp[a]exp[b] exp[x + y] Steve Luttrell "Karim Mehadhebi" <Karim.Mehadhebi at wanadoo.fr> wrote in message news:ca6igm$j2s$1 at smc.vnet.net... > Dear All, > > I can not understand why Mathmatica refuses to apply rules which transform > exponenials of sums into product of exponentails. > > For instance, > f[x+y] /. f[a_ + b_]-> f[a] f[b] returns f[x] f[y], > whereas > Exp[x+y] /. Exp[a_+b_]->Exp[a]Exp[b] still returns Exp[x+y] > > Any help greatly appreciated > > > > >