Larsen, P O and von Ins, M. The rate of growth in scientific publication and the decline in coverage provided by Science Citation Index. In Scientometrics, (84) 3: 575-603, Year 2010.
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: 265--269, Wiley Subscription Services, Inc., A Wiley Company, Year 1973.