Moroz, V., Vignoli, A. and Zabreĭko, P. P. (1998) “On the three critical points theorem”, Topological Methods in Nonlinear Analysis, 11(1), pp. 103–113. Available at: https://apcz.umk.pl/TMNA/article/view/TMNA.1998.006 (Accessed: 26 April 2024).