J. Bezdek, W. Li, Y. Attikiouzel, and M. Windham. Soft Computing - A Fusion of Foundations, Methodologies and Applications, 1 (4):
166--179(December 1997)
G. Salton, and C. Buckley. SIGIR '88: Proceedings of the 11th annual international ACM SIGIR conference on Research and development in information retrieval, page 147--160. New York, NY, USA, ACM Press, (1988)