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MathGroup Archive 1997

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Nonlinear Fit Problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg7221] Nonlinear Fit Problem
  • From: peter.geissler at iwr.uni-heidelberg.de
  • Date: Fri, 16 May 1997 02:31:24 -0400 (EDT)
  • Organization: InterNetNews at News.BelWue.DE (Stuttgart, Germany)
  • Sender: owner-wri-mathgroup at wolfram.com

Hi everybody,

I have to fit the function

A / ((x-B)^C)   (x = independent variable, A,B,C parameters)

to a given dataset.

I tried to use the NonlineaFit function from the
Statistics'NonlinearFit' package, but did not not succeed.

I tried

data = {{x1,y1},{x2,y2},......} with {x,y} beeing my actual data

NonlinearFit[data, A / ((x-B)^C), x, {A,B,C}]


I have two major problems:

1) the fit does not converge to reasonable parameters.

2) I cannot specify any start values. I tried to give them
as {{A,1000.0},{B,0.0},{C,1.0}} and similiar forms, but
always got error messages concerning the parameters.

3) mathematica shows kind of a strange behavior when
repeating the fit operation, sometimes it keeps old values,
sometimes it reports different error messages or does not
succeed on simpler fits which worked before. 

Any help welcome,
Peter

--------------------------------------------------------------------------------------
Peter Geissler
IWR - Univ. of Heidelberg
peter.geissler at iwr.uni-heidelberg.de
-------------------------------------------------------------------------------------




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