1. https://en.wikipedia.org/wiki/Barter
2. https://en.wikipedia.org/wiki/Late_Bronze_Age_collapse
3.https://en.wikipedia.org/wiki/Dark_Ages_(historiography)
4. Humphrey, C. (1985). Barter and Economic Disintegration. Man, 20(1), new series, 48-72. doi:10.2307/2802221
https://www.jstor.org/stable/2802221?seq=1#page_scan_tab_contents
5. Forrester, Rochelle, History of Writing and Record Keeping (October 31, 2016). How Change Happens: A Theory of Philosophy of History, Social Change and Cultural Evolution, (2009), Best Publications Limited. Available at SSRN: https://ssrn.com/abstract=2862739 or http://dx.doi.org/10.2139/ssrn.2862739
6. https://archive.org/stream/Debt-The_First_5000_Years#page/n27/mode/2up
7. History Begins at Sumer (Philadelphia, 1956), Thirty Nine Firsts in Recorded History, by Samuel Noah Kramer, in 404 searchable pdf pages.
http://www.radiantlunatic.com/wp-content/uploads/2013/10/HistoryBeginsAtSumer.pdf
8. K. WICKSELL (1936), Interest and Prices: A Study of the Causes Regulating the Value of Money, translated by Richard Kahn, (1962 reprint, Augustus Kelley, New York).
9. http://www-ceel.economia.unitn.it/staff/leijonhufvud/files/wick.pdf
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