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MathGroup Archive 2002

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Problem using NonlinearFit/Regress?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg33858] Problem using NonlinearFit/Regress?
  • From: redrooz at yahoo.com (Red Roo)
  • Date: Sat, 20 Apr 2002 02:49:48 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

I am trying to estimate the parameters {a,b} in the
function: x / (1 + a (x-1) + a b x (x-1)) using 
NonlinearFit[...].

The model is known to be a CONCAVE function with 0 <
a,b < 1 and b < a (in general).  NonlinearFit/Regress
always produces negative parameters (i.e., a convex
function); even if I try to condition them to be
positive e.g., {a, 0, 1} and  {b, 0, 1}.

Is there something I am missing about the use of 
NonlinearFit[...] for the above case?

--njg


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