Re: FindFit bug
- To: mathgroup at smc.vnet.net
- Subject: [mg116087] Re: FindFit bug
- From: Albert Retey <awnl at gmx-topmail.de>
- Date: Tue, 1 Feb 2011 06:54:49 -0500 (EST)
- References: <ii5rhr$fh4$1@smc.vnet.net>
Hi,
> 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.
Which version are you using? For me it gives a warning with version 8
but not with version 7. In both cases using Method "NMinimize" yields a
correct result:
fit = FindFit[dat, a Sin[w t + f], {a, w, f}, t,
Method -> "NMinimize"]
It is interesting to see that version 8 finds the parameters with
negative signs when no additional constraints are given (which is of
course also a correct result and you can enforce the positive signs with
an additional constraint)...
hth,
albert