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MathGroup Archive 2004

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Re: Problem with FullSimplify in Version 5: Rationals are converted to Reals

  • To: mathgroup at smc.vnet.net
  • Subject: [mg46305] Re: [mg45482] Problem with FullSimplify in Version 5: Rationals are converted to Reals
  • From: oberfeld <oberfeld at uni-mainz.de>
  • Date: Fri, 13 Feb 2004 21:57:34 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Dear list,

some weeks ago, I posted a question concerning FullSimplify in Ma 5.0 
which seems to occur because Rationals are converted to Reals.

Thanks to those who suggested work-arounds! Unfortunately, I was not 
able to resolve my problem following the suggestions. Wolfram support 
meanwhile confirmed this to be a problem of version 5.0.0 and said that 
the problem were   resolved in Ma 5.0.1. However, after installing the 
update I had to realize that this is *not correct*.

So, for all who are interested, here comes the 'full problem' (and its 
solution) again:

I have to solve a rather nasty expr for a variable deltaL:

expr=1/Sqrt[2] ==
   (1/2)*(1 + (1/2)*(-1 - Erf[(-(Log[10000000000]/Log[10]) - Log[10^(-12 
+ (20 + deltaL)/10)]/Log[10])/
          (Sqrt[2]*Sqrt[0.25302502757884177 + (0.6089140226261116 - 
0.005294904544574884*(20 + deltaL))^2])])) +
    (1/4)*(1 + Erf[(Log[10000000000]/Log[10] + Log[10^(-12 + (20 + 
deltaL)/10)]/Log[10])/
        (Sqrt[2]*Sqrt[0.25302502757884177 + (0.6089140226261116 - 
0.005294904544574884*(20 + deltaL))^2])]);

In Ma 3+4, the following straightforward expression works:

Solve[FullSimplify[expr],deltaL]

In Ma 5, however, the same expression produces the all-too-well-known 
message "Solve::tdep: The equations appear to involve the variables to 
be solved for in an essentially non-algebraic way."

Switching to Simplify (instead of FullSimplify), as Andrzej & Bob 
suggested, does not help (at least on my machine).

The only solution I found (thank's for the idea, Bob!), was to use 
Rationalize:

N[Solve[Simplify[Rationalize[expr,0]],deltaL]]
Out:={deltaL->3.7999}

Note, that *FullSimplify* still does not work!!

N[Solve[FullSimplify[Rationalize[expr,0]],deltaL]]



Would be no problem were it not for many of such expressions lingering 
in my code, so that I have to step through it and insert that 
"Rationalize magic" each and every time... ;-(

Best wishes

Daniel


-------------------------------------
Dipl. Psych. Daniel Oberfeld-Twistel
University of Mainz
Institute of Psychology
Experimental Psychology

Staudingerweg 9
55099 Mainz
Germany

T.   ++49 (0) 6131 39 22423
Fax. ++49 (0) 6131 39 22480


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