J.-H. He, “A parametrized variational principle of nonlinear piezoelectricity”, Comp. Assist. Methods Eng. Sci., vol. 10, no. 3, pp. 263–269, Jan. 2023, Accessed: Jun. 30, 2025. [Online]. Available: https://cames.ippt.pan.pl/index.php/cames/article/view/1074