== Eksterne kilder/henvisninger ==
{{Commonscat|Inca Empire}}
* [http://www.archaeology.org/online/features/llactapata/ November 18, 2003, archaeology.org: Llactapata: A Big Inca Discovery, or Not?]
* [http://www.freerepublic.com/focus/f-news/1070601/posts Incan Counting System Decoded? By Rossella Lorenzi, Discovery News] Citat: "...Learn how to add 9+7 on the [[yupana abacus]]...According to De Pasquale, the circles in the cells are nothing but the first numbers of the Fibonacci series, in which each number is a sum of two previous: 1, 2, 3, 5...The abacus would then work on a base 40 numbering system..."
Filosofi Historie Tidligere_riger,_stater,_protektorater,_kolonier_og_lande