Corvellec, Jean-Noël, Marco Degiovanni, and Marco Marzocchi. “Deformation Properties for Continuous Functionals and Critical Point Theory”. Topological Methods in Nonlinear Analysis 1, no. 1 (March 1, 1993): 151–171. Accessed April 25, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.1993.012.