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MathGroup Archive 2005

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Complex Oddity

  • To: mathgroup at
  • Subject: [mg57455] Complex Oddity
  • From: "John Reed" <nospamjreed at>
  • Date: Sat, 28 May 2005 05:39:32 -0400 (EDT)
  • Sender: owner-wri-mathgroup at

I was trying to separate the real and imaginary parts of a complicated 
expression, and ended up with something strange.  Here is a short version of 
what happened.

Let z = x + I y, then realPart = z /. {Complex[a_,b_]->a} gives realPart = 
x.  Great!

Now, try imagPart = z /. {Complex[a_,b_]->b}  returns with imagPart = x + y. 

In my original expression, it was harder to see, but the same error was 
occuring.  What I tried first was using Re[z] and Im[z], but then I have to 
work with Im[y] and Im[x].  It seems to me two things need to be done here. 
First, be able to assign variables so that they always stay real or else 
indicate an error is occuring if they turn out to be complex, and second do 
something to avoid errors like the above.  I have to say that I don't trust 
Mathematica's answers as much as I did before this came up. Now I feel like 
I better have a good idea of what the answer is before I  trust Mathematica.

John Reed 

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