LI, Chun, AGARWAL, Ravi P. and OU, Zeng-Qi. Existence of three nontrivial solutions for a class of fourth-order elliptic equations. Topological Methods in Nonlinear Analysis. Online. 22 April 2018. Vol. 51, no. 2, pp. 331 - 344. [Accessed 3 July 2025].