"salinan arkib"(PDF). Diarkibkan daripada yang asal(PDF) pada 2008-09-11. Dicapai pada 2010-02-20.
wikipedia.org
en.wikipedia.org
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N. Gregory Mankiw (4th Edition 2007). Principles of Economics. ISBN0-32-422472-9. Check date values in: |year= (bantuan)