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

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