Re: NonlinearRegression

*To*: mathgroup at smc.vnet.net*Subject*: [mg25152] Re: NonlinearRegression*From*: "Bill Bertram" <wkb at ansto.gov.au>*Date*: Sun, 10 Sep 2000 21:25:44 -0400 (EDT)*Organization*: Australian Nuclear Science and Technology Organisation*References*: <8p9kit$1rm@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

David Pires <l45337 at alfa.ist.utl.pt> wrote in message news:8p9kit$1rm at smc.vnet.net... > I want to fit my data with a function such as: > f(x)=Sqrt(a-b*x), where a and b are the parameters I want to adjust and x > is the variable. The problem is that there is only a given range of x for > that f(x) has real values. When I use the NonlinearRegression (or > NonlinearFit) command, Mathematica gives error messages and aborts the > fitting process. > > I'd like to know if someone knows how to set the range of x when using > that command. Hi, Here is a method I have used in the past. Instead of fitting to f(x) = Sqrt[a-b*x], define the fitting function as f[a_,b_,x_] := If[a-b*x > 0, Evaluate[Sqrt[a-b*x]], 0] as in the following example, << Statistics`NonlinearFit` data = Table[{x, Sqrt[1 - 0.2x]}, {x, 0, 4, 0.3}] func[a_, b_, x_] := If[a - b x > 0, Evaluate[Sqrt[a - b x]], 0]; NonlinearRegress[data, func[a, b, x], {x}, {a, b}] This seems to work fine. Cheers, Bill