Constrained spline smoothing

Several results on constrained spline smoothing are obtained. In particular, we establish a general result, showing how one can constructively smooth any monotone or convex piecewise polynomial function (ppf) (or any q-monotone ppf, q ≥ 3, with one additional degree of smoothness) to be of minimal defect while keeping it close to the original function in the double-struck L_p-(quasi)norm. It is well known that approximating a function by ppfs of minimal defect (splines) avoids introduction of artifacts which may be unrelated to the original function; thus it is always preferable. On the other hand, it is usually easier to construct constrained ppfs with as few requirements on smoothness as possible. Our results allow us to obtain shape-preserving splines of minimal defect with equidistant or Chebyshev knots. The validity of the corresponding Jackson-type estimates for shape-preserving spline approximation is summarized; in particular, we show that the double-struck L_p-estimates, p ≥ 1, can be immediately derived from the double-struck L_∞-estimates.