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Re: Nonlinear fit for a non-differentiable function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg43154] Re: Nonlinear fit for a non-differentiable function
  • From: Paul Abbott <paul at physics.uwa.edu.au>
  • Date: Thu, 14 Aug 2003 07:06:39 -0400 (EDT)
  • Organization: The University of Western Australia
  • References: <bhd94f$rn9$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

In article <bhd94f$rn9$1 at smc.vnet.net>,
 Ahmed Sayeed <Ahmed.Sayeed at Tu-Ilmenau.De> wrote:

>    I have to carry out a nonlinear fit where the fit function is 
>    non-differentiable w.r.t. one or
> more of the parameters, because I have to take the absolute value in the 
> definition of the function.
> For example, f[x_,a_,b_]:=a*Sin[Abs[b]*x], where a and b are fit parameters.  
> NonlinearRegress
> fails, giving the following message:
> NonlinearRegress::"unevldr": "One or more derivatives of the model with \
> respect to parameters did not evaluate."
>    I shall be grateful if someone could suggest a way around this problem.

One way might be to replace Abs[b] by Sqrt[b^2] (valid for b real) which 
is differentiable.

Cheers,
Paul

-- 
Paul Abbott                                   Phone: +61 8 9380 2734
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