O. Davydov and W. P. Yeo, Refinable C2 piecewise quintic polynomials on Powell-Sabin-12 triangulations, J. Comput. Appl. Math., 240 (2013), 62-73. doi:10.1137/11082539X

Abstract: We present a construction of nested spaces of $C^2$ macro-elements of degree 5 on triangulations of a polygonal domain obtained by uniform refinements of an initial triangulation and a Powell-Sabin-12 split.

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