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MathGroup Archive 1998

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Re: Re:Nonlinear fit to inverse power law data plot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg14149] Re: [mg14115] Re:Nonlinear fit to inverse power law data plot
  • From: "Jens-Peer Kuska" <kuska at linmpi.mpg.de>
  • Date: Mon, 28 Sep 1998 18:57:15 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Debbie,

You can fit 
y=c*x^(-n) simply with a linear fit (Fit[]) if You use

{Log[x],Log[y]}

instead of the original data pairs {x,y}

Your model say that Log[y]==Log[c]-n*Log[x]

say Your data are named dataset
dataset=Table[{x,0.2*x^(-1.5)},{x,0.5,10,0.1}];

fitPara=Fit[dataset /. {x_?NumericQ,y_}:>{Log[x],Log[y]},{1,lnx},lnx] /.

    a_. +b_.*lnx :> {E^a,b}

will return a list of c and -n.

Hope that helps
  Jens


-----Original Message-----
From: jleddon <jleddon at cyberramp.net> To: mathgroup at smc.vnet.net
Subject: [mg14149] [mg14115] Re:Nonlinear fit to inverse power law data plot


>
>Hello,
>I have a set of data that when plotted, produces a curve that is
>definitely of a inverse-power-law form;
>
> y=constant*x^-n       where n may be a non-integer.
>
>I have tried the nonlinear fit function with model function forms,
>however, I have not been able to get a non-integer value for n.
>
>Does anyone have any ideas?
>
>Thanks in advance for your help!
>
>Regards,
>Debbie Leddon
>
>
>



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