Gutik, Oleg, and Anatolii Savchuk. “On the Monoid of Cofinite Partial Isometries of $\qq{N}^n$ With the Usual Metric”. Proceedings of the International Geometry Center 12, no. 3 (December 28, 2019): 51-68. Accessed March 28, 2024. //journals.ontu.edu.ua/index.php/geometry/article/view/1553.