Omarova, B., and Z. Sartabanov. “On Multiperiodic Solutions of Perturbed Nonlinear Autonomous Systems With the Differentiation Operator on a Vector Field”. Eurasian Mathematical Journal, vol. 12, no. 1, Mar. 2021, pp. 68-81, doi:10.32523/2077-9879-2021-12-1-68-81.