Kalybay, A.A., and A.O. Baiarystanov. “Differential Inequality and Non-Oscillation of Fourth Order Differential Equation”. Bulletin of the Karaganda University. Mathematics Series 104, no. 4 (December 30, 2021): 103–109. Accessed December 8, 2025. https://mts.buketov.edu.kz/mathematics-vestnik/article/view/478.