RE: How to substitute a function?
- To: mathgroup at smc.vnet.net
- Subject: [mg94128] RE: [mg94082] How to substitute a function?
- From: "David Park" <djmpark at comcast.net>
- Date: Fri, 5 Dec 2008 05:33:28 -0500 (EST)
- References: <29071960.1228395614492.JavaMail.root@m02>
Perhaps your 'miracle' answer was all too typical of miracles. Try this: expr = \!\( \*SubscriptBox[\(\[PartialD]\), \(x\)]\ \(y[x]\)\) + a*y[x]^2 expr/.y->Function[x,f[x]Exp[-x]]//Simplify giving... a y[x]^2+(y^\[Prime])[x] E^(-2 x) (-E^x f[x]+a f[x]^2+E^x (f^\[Prime])[x]) David Park djmpark at comcast.net http://home.comcast.net/~djmpark/ From: Alexei Boulbitch [mailto:Alexei.Boulbitch at iee.lu] Dear crew, I faced a difficulty when trying to substitute a newly represented function into an expression containing a sum of differential and algebraic terms. The difficulty is namely, that Mathematica 6 substitutes the new representation into the algebraic, but not into the differential part. For example: (* This is the definition of a simple example of such an expression *) expr = \!\( \*SubscriptBox[\(\[PartialD]\), \(x\)]\ \(y[x]\)\) + a*y[x]^2; (* Here I substitute a new representation of the function into the \ above expression *) (* What I expect to get is the following expression: -Exp[-x]*f[x] + \ Exp[-x]*f=B4[x] + a \[ExponentialE]^(-2 x) f[x]^2 *) (* Instead I get something else. Please have a look and check: *) expr /. y[x] -> Exp[-x]*f[x] (* This is another approach I could think of with even worse result. *) \ (* Note however, that within this approach a miracle happened: once \ it *) (* worked as expected (i.e. the substitution of both terms has been \ performed as desired, but only once *) FullSimplify[expr, y[x] == Exp[-x]*f[x]] Do you have idea of how to instruct Mathematica to make the substitution everywhere? Thank you in advance, Alexei -- Alexei Boulbitch, Dr., Habil. Senior Scientist IEE S.A. ZAE Weiergewan 11, rue Edmond Reuter L-5326 Contern Luxembourg Phone: +352 2454 2566 Fax: +352 2454 3566 Website: www.iee.lu This e-mail may contain trade secrets or privileged, undisclosed or otherwise confidential information. If you are not the intended = recipient and have received this e-mail in error, you are hereby notified that any review, copying or distribution of it is strictly prohibited. Please = inform us immediately and destroy the original transmittal from your system. = Thank you for your co-operation.