Nikola Obreshkoff (1896-1963). Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, [S. l.], v. 89, p. 7, 1997. Disponível em: https://annual.uni-sofia.bg/index.php/fmi/article/view/347. Acesso em: 15 dec. 2025.