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Extrapolate spline


g = fnxtr(f,order)
g = fnxtr(f)



g = fnxtr(f,order) returns a spline of order order that extrapolates the spline f. g equals f on its basic interval, but g is a polynomial of the given order outside that interval. g satisfies at least order smoothness conditions at the ends of the basic interval of f, that is, at the new breaks. It is most useful to use a positive value of order that is smaller than the order of f.

g = fnxtr(f) uses an extrapolation order equal to 2. It is equivalent to fnxtr(f,2).


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Create a cubic smoothing spline on the unit interval.

x = rand(1,21);
spline = csaps(x,x.^3); 

Create an extrapolating spline of order 2.

order = 2;
extrSpline = fnxtr(spline,order);

Plot the original spline together with the extrapolating spline.

fnplt(spline,[-.5 1.4])
hold on
fnplt(extrSpline,[-.5 1.4])
legend('Cubic smoothing spline','Extrapolating spline')
hold off

Create and plot a bivariate B-spline.

spline = spmak({0:3,0:4},1);

Create an extrapolating spline. To extrapolate in the first variable only, specify a negative integer as the extrapolation order in the second variable.

order = [3,-1];
extrSpline = fnxtr(spline,order);

Input Arguments

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Spline to extrapolate, specified as a structure. f must be a spline in B-form, BBform, or ppform.

Data Types: struct

Order of extrapolating spline, specified as an integer or a vector of integers.

If order is zero, then the extrapolating spline describes the same spline as fn2fm(f,'B-'), but is in ppform and has a larger basic interval.

If order is at least as large as the order of f, then the extrapolating spline is the same spline as gn2fm(f,'pp'), but uses two more pieces and has a larger basic interval.

If f is m-variate, then order can be a vector with m elements, where order(i) is the order used to extrapolate in the i-th variable. To exclude the i-th variable from being used in the extrapolation, specify order(i) as a negative integer.

Example: 1

Output Arguments

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Spline in ppform, returned as a structure with these fields.

Form of the spline, returned as pp. pp indicates that the spline is given in piecewise polynomial form.

Knot positions of the spline, returned as a vector or as a cell array of vectors for multivariate data. Vectors contain strictly increasing elements that represent the start and end of each of the intervals over which the polynomial pieces are defined.

Coefficients of polynomials for each piece, returned as a matrix or as an array for multivariate data.

Number of polynomial pieces describing the spline, returned as a scalar or as a vector of numbers of pieces in each variable for multivariate data.

Order of the polynomial function describing each polynomial piece of the spline, returned as a scalar or as a vector containing the order in each variable for multivariate data.

Dimensionality of the target function, returned as a scalar.

See Also

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Introduced in R2006a