V. A. Dobrynskii, “On a smooth solution of a nonlinear periodic boundary-value problem”, Ukr. Mat. Zhurn., vol. 51, no. 11, pp. 1574–1576, Nov. 1999, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4760