LOOK AT:

(a) Jiawei Han and Micheline Kambe. Data Mining: Concepts and Techniques, The Morgan Kaufmann Series in Data Management Systems, Jim Gray, Series Editor Morgan Kaufmann Publishers, August 2000. 550 pages. ISBN 1-55860-489-8
CHAPTER 8: Cluster Analysis, Section 8.4.1


(b) MacQueen, J, Some Methods for Classification and Analysis of Multivariate Observations, 5th Berkeley Symposium on Mathematical Statistics and Probability, 1967.