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nonlinear regression curve fitting


	Does anyone know of a package to do nonlinear regression curve fit to a function, possibly using Marquardt's algorithm?
	I am interested in fitting data to an equation of the form y=f(y,x) or
z=f(x,y). Below is the equation I wish to fit.

The equation:
          _                                           _
         |                                             |
    [L]  |        F1                        F2         |
Y= ----- | -------------------  +  ------------------- |
     K   | 1 + [I]/K1 + [L]/K      1 + [I]/K2 + [L]/K  |
         |_                                           _|
        
There are 4 parameters to be determined: K, K1, K2, F1 (since F1 + F2 =1, F2
is not a parameter. Our raw data is obtained as radioactive counts versus the 
independent variable, [I]. The counts are used to calculate both Y and [L] thus
our dependent variable appears on both sides of the equation. Y=(T-n)/r and 
[L]=q-T. As written above the equation is a function of two variables [I] and
[L] but it can be recast as function of one T=f(T([I]),[I]).

Sincerely

Walter K. Vogel
vogelw at bionette.cgrb.orst.edu




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