GRALA, Hieronim. The role of Russia in the wars between Byzantium and Bulgars on the turn of 12th and 13th centuries. Balcanica Posnaniensia. Acta et studia, [S. l.], v. 2, p. 125–132, 1985. DOI: 10.14746/bp.1985.2.9. Disponível em: https://pressto.amu.edu.pl/index.php/bp/article/view/54820. Acesso em: 4 oct. 2026.