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