GRIGONIS, Jonas. Engels and Natural Sciences. Problemos, [S. l.], v. 7, p. 14–17, 1971. DOI: 10.15388/Problemos.1971.7.5806. Disponível em: https://www.journals.vu.lt/problemos/article/view/5806. Acesso em: 6 dec. 2025.