Re: NonlinearModelFit and assumptions on fit parameters
- To: mathgroup at smc.vnet.net
- Subject: [mg128213] Re: NonlinearModelFit and assumptions on fit parameters
- From: Alexei Boulbitch <Alexei.Boulbitch at iee.lu>
- Date: Tue, 25 Sep 2012 04:38:56 -0400 (EDT)
- Delivered-to: l-mathgroup@mail-archive0.wolfram.com
- Delivered-to: l-mathgroup@wolfram.com
- Delivered-to: mathgroup-newout@smc.vnet.net
- Delivered-to: mathgroup-newsend@smc.vnet.net
Hi
I have a set of data (x, y) that I can succesfully fit a nonlinear function to using NonlinearModelFit:
data = {{1, 1}, {2, 2}, {3, 3.2}};
fitFuncExactNoLosses[a_, b_, x_] := a*x^2 + b + x;
nlm = NonlinearModelFit[data, fitFuncExactNoLosses[a, b, x],
{
{a, 1},
{b, 1}},
x]
However, the paramter "b" comes out negative and it *must* be positive. Is there a way to utilize assumptions such that b is constrained to be grater than zero?
Best,
Niels.
Hi, Niels,
Try this:
ff = FindFit[data, {a*x^2 + b + x, b > 0.05}, {{a, 1}, {b, 1}}, x]
Show[{ListPlot[data, PlotStyle -> Red],
Plot[(a*x^2 + b + x) /. ff, {x, 0, 3}]}]
Have fun, Alexei
Alexei BOULBITCH, Dr., habil.
IEE S.A.
ZAE Weiergewan,
11, rue Edmond Reuter,
L-5326 Contern, LUXEMBOURG
Office phone : +352-2454-2566
Office fax: +352-2454-3566
mobile phone: +49 151 52 40 66 44
e-mail: alexei.boulbitch at iee.lu