http://www.worldeconomics.com/Data/MadisonHistoricalGDP/Madison%20Historical%20GDP%20Data.efp
http://datahub.io/dataset/econonomic-history-gdp-historical-estimates
http://www.ggdc.net/maddison/maddison-project/home.htm
http://www.theworldeconomy.org/
http://en.wikipedia.org/wiki/List_of_regions_by_past_GDP_%28PPP%29_per_capita
