Reich, Simeon, and Alexander J. Zaslavski. “Well-Posedness and Porosity in Best Approximation Problems”. Topological Methods in Nonlinear Analysis 18, no. 2 (December 1, 2001): 395–408. Accessed April 23, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2001.041.