Todorov, V., M. Apostolova, E. Brankova, S. Zlateva, V. Teneva, and D. Khristov. “Study of the Scientific Work by Quantitative Methods: Some Results on Academician Nikola Obreshkoff’s Works”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 89, Dec. 1997, pp. 117-2, https://ftl5.uni-sofia.bg/index.php/fmi/article/view/361.