Fihel, I.R., and O.V. Gutik. “On the Closure of the Extended Bicyclic Semigroup”. Carpathian Mathematical Publications 3, no. 2 (December 29, 2011): 131–157. Accessed May 7, 2024. https://scijournals.pnu.edu.ua/index.php/cmp/article/view/3075.