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Re: FindFit bug

  • To: mathgroup at smc.vnet.net
  • Subject: [mg116075] Re: FindFit bug
  • From: "Kevin J. McCann" <Kevin.McCann at umbc.edu>
  • Date: Tue, 1 Feb 2011 06:52:31 -0500 (EST)
  • References: <ii5rhr$fh4$1@smc.vnet.net>

The problem is that for a nonlinear fit there may be local minima. You 
need to give FindFit a reasonably good initial guess. For example

fit = FindFit[dat, a Sin[w t + f], {{a, 2}, w, f}, t]

starts with a at 2, and the result gives the correct answer. I usually 
use Manipulate to get a good first guess on the parameters for a 1d 
function like this.

Kevin

On 1/31/2011 3:23 AM, Igor wrote:
> Hello,
> It seems that FindFit cannot fit a sine function.
> It produces no warnings and gives a totally
> wrong answer:
>
> dat = Table[{t, 3 Sin[3 t + 1]}, {t, -3, 3, 0.1}];
>
> fit = FindFit[dat, a Sin[w t + f], {a, w, f}, t]
>
> Show[ListPlot[dat], Plot[a Sin[w t + f] /. fit, {t, -3, 3}]]
>
> Output: {a ->  0.599211, w ->  1.51494, f ->  3.80421}
>
> At the same time it fits Sin[3t+1] just fine.
>


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