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WikiPedia: Số_may_mắn http://mathworld.wolfram.com/HappyNumber.html http://mathworld.wolfram.com/SadNumber.html http://mathsball.blogspot.com.es/2013/07/football-... http://mathforum.org/library/drmath/view/55856.htm... http://www.maths.leeds.ac.uk/pure/staff/allenby/al... https://web.archive.org/web/20060204094653/http://...