Extracting Real Parts from NN Expression

*To*: mathgroup at smc.vnet.net*Subject*: [mg51149] Extracting Real Parts from NN Expression*From*: carlos at colorado.edu (Carlos Felippa)*Date*: Wed, 6 Oct 2004 04:34:31 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

Thank you very much to those who replied to early posts on the subject title and help me fix several mistakes. Finally got a robust extraction function that has worked on a series of problems: RealPart[c_]:=ComplexExpand[(c+Conjugate[c])/2,TargetFunctions->{Re,Im}]; I used this in expressions such as Solve[RealPart[expr]==0,{unknowns}], in which expr is a complicated nonnumeric complex-valued expression, to solve some stability transition problems in C&G. (If the real part becomes positive stability is lost). To see what happens with built-in Re[], a simple example is instructive: expr=x^2-a^2+2*I*x*y; Print[ Solve[Re[expr]==0,x] //InputForm]; Solve returns unevaluated Print[ Solve[RealPart[expr]==0,x] //InputForm]; {{x -> -a}, {x -> a}} Note: a similar imaginary part extractor ImagPart[c_]:=ComplexExpand[-I*(c-Conjugate[c])/2,TargetFunctions->{Re,Im}]; is still iffy, but I dont need it for my application. It could certainly be improved.

**Follow-Ups**:**Re: Extracting Real Parts from NN Expression***From:*Murray Eisenberg <murray@math.umass.edu>

**Re: Extracting Real Parts from NN Expression***From:*Andrzej Kozlowski <akoz@mimuw.edu.pl>