WIDUCH, Justyn Henryk. Lothar Heiser, Die Responsa ad consulta Bulgarorum des Papstes Nikolaus I (858-867). Śląskie Studia Historyczno-Teologiczne, [S. l.], v. 13, p. 379–382, 1980. Disponível em: https://journals.us.edu.pl/index.php/ssht/article/view/22070. Acesso em: 5 grudz. 2025.