Gutik, O., and A. Savchuk. “On the Monoid of Cofinite Partial Isometries of $\qq{N}^n$ With the Usual Metric”. Proceedings of the International Geometry Center, Vol. 12, no. 3, Dec. 2019, pp. 51-68, doi:10.15673/tmgc.v12i3.1553.