HASS, Joel; KIRBY, Robion. Characterizing the 4-sphere, S^4. Journal of Open Mathematical Problems, [S. l.], v. 1, n. 1, p. 52–68, 2025. Disponível em: https://jomprob.org/index.php/jomp/article/view/Vol-1Issue-1Paper-3. Acesso em: 1 jun. 2026.