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https://issues.apache.org/jira/browse/MATH-844?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13434999#comment-13434999
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Luc Maisonobe commented on MATH-844:
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In this case, the guesser fails because the function in really far from an
harmonic function. It is a triangular periodic function with amplitude +/-3 and
period 12, and all sample points are taken as integer abscissa, so values all
belong to the integer subset {-3, -2, -1, 0, 1, 2, 3}.
This is an (interesting) ill-conditioned case. As small integers are
represented exactly even as primitive doubles, all computations are exact and
one of the intermediate parameters (c2 in the source code) is a perfect 0
(there are no approximation here, the result is exact), and we divide by c2.
I would suggest to raise an exception here, and to store this as a junit test
for failure mode.
> "HarmonicFitter.ParameterGuesser" sometimes fails to return sensible values
> ---------------------------------------------------------------------------
>
> Key: MATH-844
> URL: https://issues.apache.org/jira/browse/MATH-844
> Project: Commons Math
> Issue Type: Bug
> Affects Versions: 3.0
> Reporter: Gilles
> Priority: Minor
> Fix For: 3.1
>
> Attachments: MATH-844.test.patch
>
>
> The inner class "ParameterGuesser" in "HarmonicFitter" (package
> "o.a.c.m.optimization.fitting") fails to compute a usable guess for the
> "amplitude" parameter.
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