CHAOHA, P.; GOEBEL, K.; TERMWUTTIPONG, I. Around Ulam’s question on retractions. Topological Methods in Nonlinear Analysis, [S. l.], v. 40, n. 1, p. 215–224, 2012. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2012.029. Acesso em: 17 jan. 2022.