Clapham, Christopher; Nicholson, James (2009). The concise Oxford dictionary of mathematics. Oxford University Press. pp. 164–165. ISBN 978-0-19-923594-0. 978-0-19-923594-0 ↩
Smart, James R. (1997), Modern Geometries (5th ed.), Brooks/Cole, p. 392, ISBN 0-534-35188-3 0-534-35188-3 ↩