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https://issues.apache.org/jira/browse/MATH-972?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13650888#comment-13650888
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oskar_hertwig commented on MATH-972:
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Hi Gilles,
First of all tanks for your quick response.
I went a little bit too fast in my bug report and it appears I screw up and
your code is ok. (The code is in test.java)
Nevertheless I am stilling interrogating myself about the object returned by
Linear Interpolator. Indeed, the function returned by a linear interpolator
should not be derivable on the entire domain (on each knots points) although a
Polynomial Spline Function is always derivable inside the definition domain.
Consequently your solution allows using the Derivative Method with no
restriction although exceptions should be raised in some points. (After test
your code returns the derivative number at right)
So I am asking to you if it wouldn’t be better that linear Interpolator returns
PiecewiseLinearFunction (I add the code is in PiecewiseLinearFunction.java)
which implements UnivariateFunction but not DifferentiableUnivariateFunction.
In another hand I am aware that my solution doesn’t allow the use of
DifferentiableUnivariateSolver.
If you find my suggestion is too purist. I have no problem for closing the
ticket.
Kinds regard.
> LinearInterpolator Misconception
> --------------------------------
>
> Key: MATH-972
> URL: https://issues.apache.org/jira/browse/MATH-972
> Project: Commons Math
> Issue Type: Bug
> Affects Versions: 3.2
> Environment: All
> Reporter: oskar_hertwig
> Priority: Trivial
> Attachments: PiecewiseLinearFunction.java, Test.java
>
> Original Estimate: 2h
> Remaining Estimate: 2h
>
> The method interpolate() of the class LinearInterpolator return a
> polynomialSplineFunction althought a piecewiseContinuousFunction have to be
> returned.
> This cause a bug. Indeed when the interpolated serie is localy constant the
> linear interpolation should lead to constant value between this points. This
> is not possible when the returned object is a PolynomialSplineFunction
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