Allergretto, Walter, and Duccio Papini. “Periodic Solutions to Nonlinear Equations With Oblique Boundary Conditions”. Topological Methods in Nonlinear Analysis 40, no. 2 (April 23, 2012): 225–243. Accessed April 27, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2012.030.