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MathGroup Archive 2004

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Re: NonlinearFit problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg52058] Re: NonlinearFit problem
  • From: Peter Pein <petsie at arcor.de>
  • Date: Tue, 9 Nov 2004 01:36:47 -0500 (EST)
  • References: <cmnabi$7sn$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Feng-Yin Chang wrote:
> Hi All,
>    Could anyone give me any suggestion for the specified  fitting function
>    f= r^a Exp[-b r]?
>    My data point was given below,
>    data={{0, 1.00002}, {2.31507, 26.4522}, {4.32033, 56.8265}, {6.63539,
>     59.6674}, {8.64066, 39.5536}, {10.9557, 21.6862}, {12.961,
>     10.1456}, {15.276, 4.39652}}
> 
>   The following way,
>    NonlinearFit[data,f,r,{a,b}], gives the error message,
>    FindFit::njnum:
>    The Jacobian is not a matrix of numbers at (a,b)={1.,1.}.
> 
>   How should I do this fitting without the problem?
> 
> 
> Feng-Yin Chang,
> Institute of Physics,NCTU,Taiwan
> 
Since the derivative of f with respect to a contains Log[r], you should 
replace the first point in data by sth. like {10^-6,1.00002}. The 
solution is quite stable for r0=10^-3 or smaller.

I get r^5.509797613785286/E^(0.9447850533338638*r)

Good luck,
Peter

-- 
Peter Pein
Berlin


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