"Guáman Poma’s Yupana and Inca Astronomy", Subhash Kak, Oklahoma State University, Stillwater, OK 74078, USA [no date, uploaded in 2014 on arXiv.org, last updated 17 Feb 2014]
Paper : http://arxiv.org/ftp/arxiv/papers/1401/1401.7637.pdf
Abstract : http://arxiv.org/abs/1401.7637
Conclusions : [...] the yupana of Guáman Poma most likely served the purpose of counting days for calendric use. [...]
