Canilang, Sara, Michael P. Cohen, Nicolas Graese, and Ian Seong. “The Closure-Complement-Frontier Problem in Saturated Polytopological Spaces”. New Zealand Journal of Mathematics 51 (August 6, 2021): 3–27. Accessed April 27, 2024. https://www.nzjmath.org/index.php/NZJMATH/article/view/151.