AgarwalP., GraceS. R., and O’ReganD. “Oscillation of Certain Fourth-Order Functional Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal 59, no. 3 (March 25, 2007): 291–313. Accessed October 7, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3308.