*To*: mathgroup@smc.vnet.net*Subject*: [mg10795] Re: NonLinearFit*From*: Paul Abbott <paul@physics.uwa.edu.au>*Date*: Thu, 5 Feb 1998 00:58:39 -0500*Organization*: University of Western Australia*References*: <6atagt$jlr$11@dragonfly.wolfram.com>

Phil Howe wrote: > I'm having a problem working with NonLinearFit, and would like some > advice. To demonstrate the problem, I'll use a very simple expression, > y==a*x^c +b, where I have assigned a=3, b=7, and c=2. > > This generates a simple data set: > data=Table[{x, 3*x^2+7 +5*Random[]},{x,0,4,.2}]; > > Now I try to use NonLinearFit. If I specify the value of the exponent, > "c", the routine seems to work. If I ask it to find a value of c, it > chokes, even if I tell it the right answer: > > NonlinearFit[data,a*x^c+b,x, {{c,2},{b,7},{a,3}}]; > > NonlinearFit::"badderiv": "The matrix of model derivatives (dimensions > \!\({3, 21}\)) includes DirectedInfinity in at least one element. Try > fitting without data points having indices in the list \!\({1}\)." The problem is that NonlinearFit computes the model derivatives, i.e., In[1]:= D[a*x^c + b, c] Out[1]= c a x Log[x] and, since your data includes 0, when NonlinearFit evaluates this model derivative at x->0 In[2]:= % /. x -> 0 Out[2]= c 0 a (-*) you get a DirectedInfinity. If you delete 0 from your data, data=Table[{x, 3*x^2+7 +5*Random[]},{x,0.2,4,.2}]; everything works fine: Cheers, Paul ____________________________________________________________________ Paul Abbott Phone: +61-8-9380-2734 Department of Physics Fax: +61-8-9380-1014 The University of Western Australia Nedlands WA 6907 mailto:paul@physics.uwa.edu.au AUSTRALIA http://www.pd.uwa.edu.au/~paul God IS a weakly left-handed dice player ____________________________________________________________________