DobrynskiiV. A. “On a Smooth Solution of a Nonlinear Periodic Boundary-Value Problem”. Ukrains’kyi Matematychnyi Zhurnal 51, no. 11 (November 25, 1999): 1574–1576. Accessed November 22, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4760.