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MathGroup Archive 1998

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Re: be aware!

  • To: mathgroup at smc.vnet.net
  • Subject: [mg14174] Re: [mg14140] be aware!
  • From: "Richard Finley" <rfinley at medicine.umsmed.edu>
  • Date: Wed, 30 Sep 1998 02:04:21 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Zvi,

The problem is occuring because of when you are substituting the values
of the parameters mu and sg.  In the first set of integrals you are
substituting after the integration has been done, which forces a
symbolic integration
 to give the result in terms of the parameters mu and sg which only then
are substituted to give the final result.  It might seem that this
would not matter, but if you look at this symbolic integration result
you will see that it involves the square root of squares of mu and sg
which then raises the problematic issue of what branch of Sqrt you
are on, since Mathematica in its generality does not assume variables
are real and positive.  If you force the issue by using PowerExpand
then you will get the correct answer....otherwise it may not as you
see from your results.  This has been a frequent issue in this news
group and many people who are not aware of their implicit assumption
that their variables are real and/or positive get unexpected results.
So indeed, one has to BE AWARE.

regards,  R. Finley




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