1. W. Klement, R. H. Willens, P. Duwez, Nature (London) 187(869) (1960).
2. J. J. Gilman, Science 208(856) (1980).
3. S. Kavesh, U.S. Patent No. 3,845,805 (1974).
4. H. Chen and D. Polk, U.S. Patent No. 3,856,513.
5. E. W. Montroll and W. W. Badger, Introduction to Quantitative Aspects of Social Phenomena, Gordon and Breach, New York (1974).