       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.

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

Out= {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.