[1] 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.

[2] Vulpe I. M.: The determination of periodic solutions to nonlinear systems via averaging method. (Russian). Differenciaľnye uravnenija, Vol. 14 (1978), 1558-1565. MR 0509514

[3] 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

[4] Brézis H., Nirenberg L.: Some first order nonlinear equations on a torus. Comm. Pure Appl. Math. 30 (1977), 1-11. DOI 10.1002/cpa.3160300102 | MR 0445558