PEKHTEREV, V. O. Structure of the semigroup $OT_n$. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 61, n. 9, p. 1240–1246, 2009. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3095. Acesso em: 6 dec. 2025.