Agarwal, P., S. R. Grace, and D. O’Regan. “Oscillation of Certain Fourth-Order Functional Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal 59, no. 3 (March 25, 2007): 291–313. Accessed April 25, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3308.