Kerimbekov, A. K., and E. F. Abdyldaeva. “Optimal Distributed Control for the Processes of Oscillation Described by Fredholm Integro-Differential Equations”. Eurasian Mathematical Journal, vol. 6, no. 2, June 2015, pp. 18–40, https://emj.enu.kz/index.php/main/article/view/606.