SeleznevaN. V. “Mathematical Modeling of Nilpotent Subsemigroups of Semigroups of Contracting Transformations of a Boolean”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 61, no. 7, July 2009, pp. 976-85, https://umj.imath.kiev.ua/index.php/umj/article/view/3072.