Dobrynskii, V. A. (1999) “On a smooth solution of a nonlinear periodic boundary-value problem”, Ukrains’kyi Matematychnyi Zhurnal, 51(11), pp. 1574–1576. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/4760 (Accessed: 18May2024).