Larsen, P O, von Ins, M, The rate of growth in scientific publication and the decline in coverage provided by Science Citation Index, in: Scientometrics 84 3 (2010), S. 575-603.

Small, Henry, Co-citation in the scientific literature: A new measure of the relationship between two documents, in: Journal of the American Society for Information Science 24 4 (1973), S. 265--269.