KHOMENKO, N. P.; YAVORSKY, E. B. Properties of $\varphi$-transformations of graphs and 2-manifolds . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 23, n. 2, p. 190–200, 1971. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8501. Acesso em: 5 dec. 2025.