W. W. Rouse Ball (1896). Mathematical recreations and problems of past and present times (3rd ed.). London and New York: Macmillan. ASINB0006AHNJU. OCLC2948122.
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W. W. Rouse Ball (1896). Mathematical recreations and problems of past and present times (3rd ed.). London and New York: Macmillan. ASINB0006AHNJU. OCLC2948122.