PENKOV, B. Nikola Obreshkoff (1896-1963). Encomium. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 89, p. 11–17, 1997. Disponível em: https://annual.uni-sofia.bg/index.php/fmi/article/view/350. Acesso em: 18 apr. 2025.