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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[Î¸]\ - \ \@\(n\^2 -
Sin[Î¸]\^2\))\)\/\((\(n\^2\) \
Cos[Î¸]\ + \ \@\(n\^2 - Sin[Î¸]\^2\))\))\)\^2 + C\)
Or in more-readable terms,
A* ((n^2 Cos[Î¸] - Sqrt[n^2 - Sin[Î¸]^2])/(n^2 Cos[Î¸] + Sqrt[n^2 -
Sin[Î¸]^2]))^2 + C
My pars are
{n, A, C}
My vars are
Î¸
I am typing
\!\(\(\(\ \)\(FindFit[dataRUnoErrorRad, A*\((\((\(n\^2\) Cos[Î¸]\ - \
\
\@\(n\^2 -
Sin[Î¸]\^2\))\)\/\((\(n\^2\) Cos[Î¸]\ + \ \@\(n\^2 -
Sin[Î¸]\^2\))\))\)\^2 \
+ C, {n, A, C}, Î¸\ ]\)\)\)
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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