Klimperová N.: Periodic solution of hyperbolic equations of the first and second order. (Czech). Thesis, Faculty of Mathematics and Physics of Charles University, Prague 1978.
 Vulpe I. M.: The determination of periodic solutions to nonlinear systems via averaging method
. (Russian). Differenciaľnye uravnenija, Vol. 14 (1978), 1558-1565. MR 0509514
 Choma G. P.: The determination of periodic solutions to nonlinear systems of the first order with distributed parameters
. (Russian). Ukrainskii Matematičeskij Žurnal, Vol. 33 (1981), 779-786. MR 0638699