O. Davydov and L. L. Schumaker, Stable approximation and interpolation with C^1 quartic bivariate splines, SIAM J. Numer. Anal.39 (2002), 1732-1748. DOI 10.1137/S0036142901384472

Abstract: We show how two recent algorithms [6,7,14] for computing $C^1$ quartic interpolating splines can be stabilized to insure that for smooth functions, they provide full approximation power with approximation constants depending only on the smallest angle in the triangulation.

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