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MathGroup Archive 2004

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Complexes, Reals, FullSimplify

  • To: mathgroup at smc.vnet.net
  • Subject: [mg48782] Complexes, Reals, FullSimplify
  • From: "Stergios J. Papadakis" <stergios.papadakis at jhuapl.edu>
  • Date: Wed, 16 Jun 2004 04:54:52 -0400 (EDT)
  • Organization: Johns Hopkins University Applied Physics Lab, Laurel, MD, USA
  • Sender: owner-wri-mathgroup at wolfram.com

Dear group,
I am trying to use expressions of the below form as boundary conditions 
in NDSolve.  I keep getting "non-numerical" errors.  I have tried a lot 
of things and reduced the problem to this:

These give different outputs:

FullSimplify[Re[(1 - 6* I)* Cos[x] - (1 + 2*I)*Sin[(1/2)* x]],Element[x, 
Reals]]

FullSimplify[Re[(1 - 6* I)* Cos[x] - (1 + 2*I)*Sin[0.5*x]],Element[x, 
Reals]]

I get:

\!\(Cos[x] - Sin[x\/2]\)

Re[(1 - 6 \[ImaginaryI]) Cos[x] - (1 + 2 \[ImaginaryI]) Sin[0.5 x]]


I think I can get my NDSolve to work if I can make the second 
FullSimplify above give me an output without an Re in it.  Mathematica 
assumes that 0.5 may have some tiny imaginary part and therefore keeps 
everything for full generality.  How do I eliminate
this?  Note that I have simplified things a lot here, the actual
expression that I will be using has many terms that all have the
form above, with many significant digits, which depend on earlier
calculations.  I have tried using Chop,

FullSimplify[Chop[Re[(1 -
   6* I)* Cos[x] - (1 + 2*I)*Sin[0.5* x]]], Element[x, Reals]]

And that does not work, I get the same result.






Thanks,
Stergios


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