How to substitute a function?
- To: mathgroup at smc.vnet.net
- Subject: [mg94082] How to substitute a function?
- From: Alexei Boulbitch <Alexei.Boulbitch at iee.lu>
- Date: Thu, 4 Dec 2008 07:16:17 -0500 (EST)
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´[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.
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