Doroshenko, V. V. “On Semigroups of Order-Preserving Transformations of Countable Linearly Ordered Sets”. Ukrains’kyi Matematychnyi Zhurnal 61, no. 6 (June 25, 2009): 723-732. Accessed April 19, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3055.