Todorov, V., Apostolova, M., Brankova, E., Zlateva, S., Teneva, V. and Khristov, D. 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, (Dec. 1997), 117–127.