Dumir, V. C., and R. J. Hans-Gill. “Markoff Type Chain for the View Obstruction Problem for Three Dimensional Cubes”. The Journal of the Indian Mathematical Society 57, no. 1-4 (December 1, 1991): 123–141. Accessed May 9, 2024. https://www.informaticsjournals.com/index.php/jims/article/view/21925.