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MathGroup Archive 2006

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Re: basic complex number question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg72114] Re: [mg72081] basic complex number question
  • From: Murray Eisenberg <murray at math.umass.edu>
  • Date: Wed, 13 Dec 2006 06:38:36 -0500 (EST)
  • Organization: Mathematics & Statistics, Univ. of Mass./Amherst
  • References: <200612110954.EAA07116@smc.vnet.net>
  • Reply-to: murray at math.umass.edu

Well actually Re and Im DO return the real and imaginary part of their 
arguments, as advertized.  It's just that Mathematica may not know from 
what you give as argument what the real and imaginary parts are.

For example:

   Im[2+3I]
3

(As expected.)  But:

   Im[x+I y]
Im[x+I y]

(the best Mathematica can do without mind-reading: perhaps you were 
thinking Im[(2I) + I (3I)]).

To obtain what you probably want, wrap Re and Im with ComplexExpand.
For example:

    ComplexExpand[Exp[x+I y]]
E^x*Cos[y] + I*E^x*Sin[y]

(result shown in InputForm).

Daniel Mazur wrote:
> My apologies for asking a question *this* basic, but here goes:
> Functions Im[] and Re[] don't really evaluate imaginary and real part of
> analytical expressions. What do I need to do to get this done?
> Thanks,
> Daniel
> 

-- 
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305


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