KULPA, Władysław. Rigid graphs of maps. Annales Mathematicae Silesianae, [S. l.], v. 2, p. 92–95, 1986. Disponível em: https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/14328. Acesso em: 8 may. 2025.