Polynomial formulation of B-spline
parr(xk, n = 3L)
Numeric 3D array, the first index runs through n+1 polynomial coefficients; the second -- through n+1 supporting intervals; and the last one through nk-n-1 B-splines (here nk=length(xk)). Knot interval of length 0 will have corresponding coefficients set to 0.
Numeric vector, knots
Integer scalar, polynomial order (3 by default)