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3. M Ascher and R Ascher, Mathematics of the Incas: code of the Quipu (Mineola, NY, 1997).
4. G Ifrah, A universal history of numbers : From prehistory to the invention of the computer (London, 1998).
5. G G Joseph, The crest of the peacock (London, 1991).
6. G Urton, The social life of numbers : A Quechua ontology of numbers and philosophy of arithmetic (Austin, TX, 1997).

7. M Ascher, Mathematical ideas of the Incas, in Native American mathematics (Austin, TX, 1986), 261-289.
8. M Ascher, The logical-numerical system of Inca quipus, Ann. Hist. Comput. 5 (3) (1983), 268-278.
9. W Burns Glynn, Calculation table of the Incas (Spanish), Bol. Lima No. 11 (1981), 1-15.
10. A Christensen, The Peruvian quipu, in History and science of knots (River Edge, NJ, 1996), 71-88.
11. D Pareja, Pre-Hispanic tools of computation : the quipu and the yupana (Spanish), Rev. Integr. Temas Mat. 4 (1) (1986), 37-56.
12. W S Sizer, Mathematical notions in preliterate societies, Math. Intelligencer 13 (4) (1991), 53-60.

MacTutor History of Mathematics
[http://www-history.mcs.st-andrews.ac.uk/HistTopics/References/Inca_mathematics.html]