DUMIR, V. C.; HANS-GILL, R. J. The View Obstruction Problems for Boxes. The Journal of the Indian Mathematical Society, [S. l.], v. 57, n. 1-4, p. 117–122, 1991. Disponível em: https://informaticsjournals.com/index.php/jims/article/view/21922. Acesso em: 19 may. 2024.