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MathGroup Archive 1998

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Re: Integrate Bug?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13283] Re: Integrate Bug?
  • From: Ed Hall <teh1m at virginia.edu>
  • Date: Fri, 17 Jul 1998 03:18:37 -0400
  • Organization: University of Virginia
  • References: <6nskhb$fch@smc.vnet.net> <6oert0$guj$13@dragonfly.wolfram.com>
  • Sender: owner-wri-mathgroup at wolfram.com

Alan Mahoney <mahoney at purdue.edu> wrote:
: Examining the difference in the terms produced, the "bug" is much easier
: to reproduce

: In[1]:= awm = Log[1-W]

: Out[1]= Log[1 - W]

: In[2]:= Integrate[D[awm,W],W]

: Out[2]= Log[-1 + W]

: But if you do the integration normally, the answer is "Log[Abs[-1 +
: W]]." Since Mathematica
: can't know what the value of W (or b Nt W in your case) is, it seems to
: assume > 1.  I would
: just use /. Log[a_] :> Log[Abs[a_]] before making your numerical
: substitutions.

: Alan

Thanks to all those who have replied, and for the simplification of the
problem statement. I can insert the assumptions concerning W into the 
arguments of Integrate but it does not change the result. 

In[9]:= Integrate[D[awm,W],W, Assumptions -> {0 < W, W < 1}]

Out[9]= Log[-1 + W]

I did the numerical substitutions to further illustrate the point, but
I'm more concerned with the difference in signs of the symbolic
results. It still seems to me Mathematica should come back with the
original  expression. 

Ed


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