Piasecki, Łukasz. “Retracting Ball onto Sphere in $BC_0(\mathbb R)$”. Topological Methods in Nonlinear Analysis 33, no. 2 (June 1, 2009): 307–313. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2009.020.