D. Kempe, J. Kleinberg, and E. Tardos. KDD '03: Proceedings of the ninth ACM SIGKDD international conference on Knowledge discovery and data mining, page 137--146. ACM Press, (2003)
D. Watts. Proceedings of the National Academy of Sciences of the United States of America, 99, page 5766--5771. (April 2002)http://www.jstor.org/view/00278424/sp020038/02x3936j/0.