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Re: FindFit bug

  • To: mathgroup at smc.vnet.net
  • Subject: [mg116086] Re: FindFit bug
  • From: Gary Wardall <gwardall at gmail.com>
  • Date: Tue, 1 Feb 2011 06:54:37 -0500 (EST)
  • References: <ii5rhr$fh4$1@smc.vnet.net>

On Jan 31, 2:23 am, Igor <igor.vol... at gmail.com> 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.

Numerical procedures are like little girls. =93When they are good they
are very very good but when they are bad they are very very bad=94. In
other words when ever you use a procedure that has a numerical
component you run the risk of not getting the correct answer. In
Mathematica 7=92  look at ?NSolve "Possible Issues"

Try their example:

poly = Expand[Product[x - i, {i, 30}]];
NSolve[poly == 0, x]
Solve[poly == 0, x]


FindFit is a  numerial procedure also and when used use with caution,

Gary Wardall


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