V. Todorov, 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, pp. 117–127, Dec. 1997.