schumaker: Schumaker Shape-Preserving Spline
This is a shape preserving spline <doi:10.1137/0720057>
which is guaranteed to be monotonic and concave or convex if the
data is monotonic and concave or convex. It does not use any
optimisation and is therefore quick and smoothly converges to a
fixed point in economic dynamics problems including value function
iteration. It also automatically gives the first two derivatives
of the spline and options for determining behaviour when evaluated
outside the interpolation domain.
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