Einstein, Albert. 20. Relativity: The Special and General Theory. New York: Henry Holt. 1920: 168. ISBN 978-1-58734-092-5.
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Tinder, Richard F. Relativistic Flight Mechanics and Space Travel: A Primer for Students, Engineers and Scientists. Morgan & Claypool Publishers. 2007: 33. ISBN 978-1-59829-130-8.Extract of page 33