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[Octave-bug-tracker] [bug #38063] optim: expfit inaccurate (incorrect)


From: Muhali
Subject: [Octave-bug-tracker] [bug #38063] optim: expfit inaccurate (incorrect)
Date: Tue, 09 Apr 2013 16:22:19 +0000
User-agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:20.0) Gecko/20100101 Firefox/20.0

Follow-up Comment #4, bug #38063 (project octave):

You are giving [-1; -1; 1; 1] as an initial guess for nonlin_curvefit, that
is, two negative exponentials, so no wonder the iteration gets stuck. That the
fit still looks ok is because -0.5exp(2*x) dominates for larger x. Using
MaxIter = 1000 and [0; 0; 1; 1] as initial guess makes it work.

I would still prefer renaming expfit, because who wants to have a function of
that name that gives largely incorrect estimates in the most simple case of y
= a*exp(b*x) with realistic noise?

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  <http://savannah.gnu.org/bugs/?38063>

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