Pashaie, Firooz. “On $L_1$-Biharmonic Timelike Hypersurfaces in Pseudo-Euclidean Space $E_1^4$”. Tamkang Journal of Mathematics 51, no. 4 (November 1, 2020): 313–332. Accessed April 25, 2024. https://journals.math.tku.edu.tw/index.php/TKJM/article/view/3188.