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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: [mg46312] Re: [mg45482] Problem with FullSimplify in Version 5: Rationals are converted to Reals
  • From: BobHanlon at aol.com
  • Date: Fri, 13 Feb 2004 21:58:06 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

If all you want is a numerical result then just use FindRoot

FindRoot[expr, {deltaL, 1}]

{deltaL -> 3.7999}

Bob Hanlon

In a message dated 2/13/04 11:57:44 AM, oberfeld at uni-mainz.de writes:


> Dear list,
>
> some weeks ago, I posted a question concerning FullSimplify in Mathematica 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 problems were resolved in Mathematica 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 Mathematica 3+4, the following straightforward expression works:
>
> Solve[FullSimplify[expr],deltaL]
>
> In Mathematica 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
>
>
>





Bob Hanlon


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