Kopachevsky, N. D., and E. V. Syomkina. “Linear Volterra Integro-Differential Second-Order Equations Unresolved With Respect to the Highest Derivative”. Eurasian Mathematical Journal, vol. 4, no. 4, Dec. 2013, pp. 64–87, https://emj.enu.kz/index.php/main/article/view/556.