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From: =?ISO-8859-1?Q?Hans-Bernhard_Br=F6ker?= <HBBroeker@t-online.de>
Newsgroups: comp.graphics.apps.gnuplot
Subject: Re: trying to get a fit to work.
Date: Wed, 08 Jun 2011 23:02:12 +0200
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On 08.06.2011 01:32, aviphysics wrote:
> So I got this HPLC peak that I am trying to fit a exponentialized
> Gaussian to. It fits decently in Mathematica but trying to fit it in
> gnuplot yields "Undefined value during funtion evaluation"

> Any ideas what the problem might be?

It's exactly what the message says it is: that function's value is not 
defined over the entire of the range you're trying to use it in.  This 
show up if you just plot this function in gnuplot:

	p [0:160] modeltwo(x) with linespoints

Note how the curve ends around x==100, in particular:

gnuplot> print modeltwo(100)
3.16514318564887e-009
gnuplot> print modeltwo(105)

gnuplot> print modeltwo(105)
                ^
          undefined value

The ultimate reason for that is that erfc() underflows for arguments as 
large as you try to use it with here.