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A Problem with the NonlinearFit?

  • To: mathgroup at
  • Subject: [mg61757] A Problem with the NonlinearFit?
  • From: "Axel Ligon" <ligonap at>
  • Date: Fri, 28 Oct 2005 03:25:41 -0400 (EDT)
  • Sender: owner-wri-mathgroup at

Deas Sirs,

concerning the NonlinearFit procedure I have a problem. I used the 
following inputs:

*Raw datas {x, y}*

a ->280.376, b->0.175479


280.376 * Exp(-0.175479x)

*Convert datas {x, ln[y]}*
dat2={{0,5.7680},{1,5.2817},{3,4.9430},{7,4.5951},{14,4.0271},{31,3.4904},{90, 0.6419}}

a->5.18756, b->0.0518199


5.18756 - 0.0518199 x

The difference between A) and B) is only the conversion of y to ln[y] 
and the equation y = a * exp(-b*x) to ln[y] = ln[a] - b*x. From this 
it follows that a = exp[ln[a]].
But I get in A) 280.376 / -0.175479 and in B) 179.031 / -0.05182 !!
Why do I get these error?? If I use Excel, I don't get these errors.
Is it an error in the programming of Mathematica, or not???

I'd be glad if you could help me.

With best regards

Axel Ligon

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