Schaaf, William L. (1 March 2016) [1999]. "Number game". Encyclopedia Britannica. Archived from the original on 2 February 2017. Retrieved 22 January 2017.
Schaaf, William L. (1 March 2016) [1999]. "Number game". Encyclopedia Britannica. Archived from the original on 2 February 2017. Retrieved 22 January 2017.