Iskandarov, S., and M. A. Temirov. “Estimations and Asymptotical Properties of Solutions of Weakly Nonlinear Volterra Intergrodifferential Equation of the Fourth Order With Delays”. Journal of Mathematics, Mechanics and Computer Science, vol. 72, no. 1, Mar. 2012, pp. 39-47, https://bm.kaznu.kz/index.php/kaznu/article/view/126.