I. V. Protasov and S. V. Slobodianiuk, “Ultrafilters on balleans”, Ukr. Mat. Zhurn., vol. 67, no. 12, pp. 1698–1706, Dec. 2015, Accessed: Apr. 13, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/2102