Genchev, T. (1997) “On the investigations of Nikola Obreshkoff connected with the regularly monotonic functions (in Bulgarian, summary in English)”, Ann. Sofia Univ. Fac. Math. and Inf, 89, pp. 23–35. Available at: https://admin.uni-sofia.bg/index.php/fmi/article/view/352 (Accessed: 15 January 2025).