A family of $C^1$ finite elements with optimal approximation properties for various Galerkin methods for 2nd and 4th order problemsJim Jr. Douglas, Todd Dupont, Peter Percell and Ridgway ScottRAIRO. Anal. numér., 13 3 (1979) 227-255DOI: https://doi.org/10.1051/m2an/1979130302271
