Mawhin, J. and Zanolin, F. (1993) “A continuation approach to fourth order superlinear periodic boundary value problems”, Topological Methods in Nonlinear Analysis, 2(1), pp. 55–74. Available at: https://apcz.umk.pl/TMNA/article/view/TMNA.1993.030 (Accessed: 22 July 2024).