LÖNNGREN, L. Валентность и изоморфизм<br>(Valency and isomorphism). Poljarnyj vestnik, [S. l.], v. 11, p. 15–27, 2008. DOI: 10.7557/6.1298. Disponível em: https://new.eludamos.org/index.php/vestnik/article/view/1298. Acesso em: 18 apr. 2025.