1. W. Ahrens. Mathematische Unterhaltungen und Spiele. Druck und Verlag Von B.G. Teubner, Leipzig, 1901.
2. Alfonso X, King of Castile and Leon. Libros del ajedrez, dados y tablas. 13th century.
3. S. Ammirato. Alberi genealogici, late 16th century. Biblioteca Riccardiana, Florence, Grandi formati 33, No. 67. Reproduced in [19], Fig. 6.
4. M. Ascher and R. Ascher. Code ofthe Quipu: A Study in Media, Mathematics, and Culture. University of Michigan Press, Ann Arbor, Michigan, 1981.
5. W. W. R. Ball. Mathematical Recreations and Essays. The MacMillan Company, New York, 1939. 11th Edition. First published in 1892.