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4. C. de Boor, “Quadratic spline interpolation, the sharpness of Lebesgue's inequality, and projectors which are close to each other, Univ. of Wisconsin Math. Res. Center, Rep. 1517, to appear.
5. Error bounds of high order accuracy for the state regulator problem via piecewise polynomial approximations;Bosarge;SIAM J. Contr.,1971