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From Phil Steitz <>
Subject Re: Bug (and fix) on Commons-Math SplineInterpolator
Date Sat, 03 Apr 2004 02:57:13 GMT
Al Chou wrote:
> --- Phil Steitz <> wrote:
>>Should be fixed now in CVS.
>>In addition to fixing the impl, I made the following changes:
>>SplineInterpolator.interpolate(double[], double[]) now returns a 
>>PolynomialSplineInterpolator (new class), which has an array of 
>>PolynomialFunctions representing the spline segments.
>>Both PolynomialSplineInterpolator and PolynomialFunction implement the new 
>>DifferentiableUnivariateRealFunction interface. 
>>PolynomialSplineInterpolator exposes its polynomials and knot point arrays 
>>as read-only properties (getters return copies).
>>I added tests to SplineInterpolatorTest (replaces InterpolatorTest) to 
>>verify that the correct coefficients are being computed (in the 
>>"degenerate" cases, testing against analytical values, for the sin case, 
>>using R as a reference) and that the PolynomialSplineFunctions give 
>>consistent values at the knot points and the polynomials "match up" (agree 
>>throgh 2 derivatives) at the knot points.
> Whew, that's a lot of work!  Not to discount any of it, but as I started
> reading the Javadoc for PolynomialSplineFunction I noticed you explicitly say
>     the first two derivatives of "adjacent" polynomials are constrained to
> agree at the knot points
> Isn't that a property more specifically of a cubic spline, or am I just
> ignorant of the definition of splines that use higher (or lower!) order
> polynomials?

Good point.  I suppose that it would be possible to define polynomial 
splines that are not smooth in the sense above.  Also, higher order 
polynomial splines may be constrained to agree through more than 2 
derivatives.  Therefore, I agree that this should be removed from the 
javadoc.  As it says later, the smoothness constraints are not enforced by 
the class in any case.  I will fix this.

Good catch.


> Al
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