LÖNNGREN, Lennart. Валентность и изоморфизм(Valency and isomorphism). Poljarnyj vestnik, Tromsø, Norway, v. 11, p. 15–27, 2008. DOI: 10.7557/6.1298. Disponível em: https://septentrio.uit.no/index.php/vestnik/article/view/1298. Acesso em: 22 apr. 2026.