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Nonlinear Fit with complex model

• To: mathgroup at smc.vnet.net
• Subject: [mg19605] Nonlinear Fit with complex model
• From: mtir at my-deja.com
• Date: Sat, 4 Sep 1999 21:09:13 -0400
• Sender: owner-wri-mathgroup at wolfram.com

Hi,

I am trying to fit a data set of real values with
a model that involves complex numbers, i.e.
  model[a,b,c,d]=Abs[<complex expression in
a,b,c,d>]^2

When I am using NonlinearRegress (or
NonlinearFit) Mathematica complains that "One or
more derivatives of the model with respect to
parameters did not evaluate" and does not return a
result.

I suspect that the reason for this is that Abs[],
Re[], Im[] etc. are not evaluated if the argument
is an expression rather than a number.

Does anyone know a way round this?

Thanks!
Sebastian

(For those interested: I am trying to calculate
the complex refractive index of a thin film from
reflectance measurements as a function of angle)


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