The convex interpolation of histogram by polynomial splines: The existence theorem Pavel Ženčák (2002) Acta Universitatis Palackianae Olomucensis. Facultas Rerum Naturalium. Mathematica
The Derivation of Cubic Splines with Obstacles by Methods of Optimization and Optimal Control. Gerhard Opfer, Hans Joachim Oberle (1987/1988) Numerische Mathematik