Larsen, P O and von Ins, M. "The rate of growth in scientific publication and the decline in coverage provided by Science Citation Index." Scientometrics 84 , no. 3 (2010): 575-603.

Small, Henry. "Co-citation in the scientific literature: A new measure of the relationship between two documents." Journal of the American Society for Information Science 24 , no. 4 (1973): 265--269.