Re: parameters problem in FindFit

• To: mathgroup at smc.vnet.net
• Subject: [mg92869] Re: parameters problem in FindFit
• From: Ray Koopman <koopman at sfu.ca>
• Date: Thu, 16 Oct 2008 05:04:59 -0400 (EDT)
• References: <gd4dl4\$bg2\$1@smc.vnet.net>

```On Oct 15, 2:37 am, gyz... at 139.com wrote:
> 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.

f[c_] := Abs[QRDecomposition[{1,#[[1]]^2/(c+#[[1]]),#[[2]]}& /@
lis][[2,-1,-1]]]

gives the square root of the residual sum of squares, minimized with
respect to a & b for a given value of c. Plotting f[c] shows a local
minimum at about -8.6, but the overall minimum appears to be at c ->
Infinity.

```

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