Todorov, Vassily, Mara Apostolova, Emilia Brankova, Stefka Zlateva, Veneta Teneva, and Dimitar Khristov. 1997. “Study of the Scientific Work by Quantitative Methods: Some Results on Academician Nikola Obreshkoff’s Works”. Ann. Sofia Univ. Fac. Math. And Inf. 89 (December):117-27. https://ftl5.uni-sofia.bg/index.php/fmi/article/view/361.