Agarwal, Ravi P., and Donal O’Regan. “An Essential Map Theory for ${\mathcal U}_c^{\kappa}$ and PK Maps”. Topological Methods in Nonlinear Analysis 21, no. 2 (June 1, 2003): 375–386. Accessed April 27, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2003.023.