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2. 2012 The Outlook for Energy: A View to 2040, 2012http://www.exxonmobil.com/energyoutlook
3. http://www-pub.iaea.org/MTCD/Publications/PDF/RDS2-32_web.pdf
4. Transmission of information by extraterrestrial civilizations;Kardashev;Sov. Astron.,1964