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MathGroup Archive 2006

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FindFit / NonlinearFit Problems

  • To: mathgroup at smc.vnet.net
  • Subject: [mg65671] FindFit / NonlinearFit Problems
  • From: "Xoedusk" <xoedusk at gmail.com>
  • Date: Wed, 12 Apr 2006 06:00:41 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Hello. I am trying to do a NonlinearFit or Findfit, but I get the
following cryptic error:

FindFit::sszero: The step size in the search has become less than the \
tolerance prescribed by the PrecisionGoal option, but the gradient is
larger \
than the tolerance specified by the AccuracyGoal option. There is a \
possibility that the method has stalled at a point which is not a local
\
minimum.

My data is
{{0.345721, 0.0351973}, {0.523381, 0.0267363}, {0.699319, 0.0149018},
{0.871647, 0.0036456}, {1.04792, 0.00167384}, {1.22115, 0.0413637},
{1.39689, 0.234294}}

My model is
\!\(A*\((\((\(n\^2\) Cos[θ]\  - \ \@\(n\^2 -
Sin[θ]\^2\))\)\/\((\(n\^2\) \
Cos[θ]\  + \ \@\(n\^2 - Sin[θ]\^2\))\))\)\^2 + C\)

Or in more-readable terms,
A* ((n^2 Cos[θ] - Sqrt[n^2 - Sin[θ]^2])/(n^2 Cos[θ] + Sqrt[n^2 -
Sin[θ]^2]))^2 + C

My pars are
{n, A, C}

My vars are
θ

I am typing
\!\(\(\(\ \)\(FindFit[dataRUnoErrorRad, A*\((\((\(n\^2\) Cos[θ]\  - \
\
\@\(n\^2 -
     Sin[θ]\^2\))\)\/\((\(n\^2\) Cos[θ]\  + \ \@\(n\^2 -
Sin[θ]\^2\))\))\)\^2 \
+ C, {n, A, C}, θ\ ]\)\)\)

Any help would be very very appreciated! I am able to get a nice fit by
hand, but i really need good numbers via mathematica.


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