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RE: parameters problem in FindFit

  • To: mathgroup at smc.vnet.net
  • Subject: [mg92864] RE: [mg92855] parameters problem in FindFit
  • From: "Richard Hofler" <rhofler at bus.ucf.edu>
  • Date: Thu, 16 Oct 2008 05:04:04 -0400 (EDT)
  • References: <200810150937.FAA11790@smc.vnet.net>

Hi,

You'll probably get many similar answers. The Mathematica Documentation
Center says this:

FindFit[data,{expr,cons},pars,vars]
finds a best fit subject to the parameter constraints cons.

So, add constraints to your function.

In[5]:= FindFit[lis, {a - (b x^2)/(c + x), {1.13 < a < 1.15,
       .0001 < b < .0004, 299 < c < 301}}, {a, b, c}, x]

Out[5]= {a -> 1.14624, b -> 0.00035099, c -> 299.942}

I hope this helps.

Richard Hofler

-----Original Message-----
From: gyzhou at 139.com [mailto:gyzhou at 139.com]
Sent: Wednesday, October 15, 2008 5:38 AM
To: mathgroup at smc.vnet.net
Subject: [mg92864] [mg92855] parameters problem in FindFit

Hi, guys :
  I get a list like this :
    lis = {{12.5, 1.146}, {32, 1.145}, {50,
   1.144}, {69, 1.1424}, {84.6,
   1.139}}
My aim is :
  FindFit[lis,
   a - (b x^2)/(c + x), {{a, 1.14}, {b, 0.0003}, {c, 300}}, x]
As we can see, I want this list to be fitted using parameters a, b, c
around 1.14, 0.0003, 300. However Mathematica gives { b, 10^8} and {c,
   10^14} far from what I expect.I set "MaxIterations" to less times,
but it doesn' t make sense.
My friends didn't give the answer either,   I am here for your advice.
Thanks in advance.



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