V. V. Doroshenko, “On semigroups of order-preserving transformations of countable linearly ordered sets”, Ukr. Mat. Zhurn., vol. 61, no. 6, pp. 723–732, Jun. 2009, Accessed: Apr. 09, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3055