DoroshenkoV. V. “On Semigroups of Order-Preserving Transformations of Countable Linearly Ordered Sets”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 61, no. 6, June 2009, pp. 723-32, https://umj.imath.kiev.ua/index.php/umj/article/view/3055.