Alymbaev, A. T., A. Bapa Kyzy, and F. K. Sharshembieva. “PERIODIC SOLUTIONS OF A SECOND-ORDER NONLINEAR VOLTERRA INTEGRO-DIFFERENTIAL EQUATION”. Advances in Differential Equations and Control Processes, Vol. 31, no. 2, May 2024, pp. 285–297, doi:10.17654/0974324324015.