Michio Kaku (1995). “Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps, and the Tenth Dimension”, p.130, Oxford University Press, USA
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”