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G. Stanley Hall (2013). “Adolescence - Its Psychology and Its Relations to Physiology, Anthropology, Sociology, Sex, Crime, and Religion (1931)”, p.428, Read Books Ltd
"'Parks and Recreation's' Nick Offerman on Mustaches and Being Tackled by Robin Givens (Q&A)". Interview with Philiana Ng, www.hollywoodreporter.com. November 15, 2011.
Felix Klein (1893). “The Evanston Colloquium: Lectures on Mathematics Delivered from Aug. 28 to Sept. 9, 1893 Before Members of the Congress of Mathematics Held in Connection with the World's Fair in Chicago at Northwestern University, Evanston, Ill”