Luyen, Duong Trong, and Mai Thi Thu Trang. “Multiple Solutions to Boundary-Value Problems for Fourth-Order Elliptic Equations”. Ukrains’kyi Matematychnyi Zhurnal 75, no. 6 (June 20, 2023): 830–841. Accessed December 8, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/6958.