“On the investigations of Nikola Obreshkoff connected with the regularly monotonic functions (in Bulgarian, summary in English)” (1997) Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, 89, pp. 23–35. Available at: https://annual.uni-sofia.bg/index.php/fmi/article/view/352 (Accessed: 17 March 2026).