Gowers, Timothy ; Barrow-Green, June; Leader, Imre . The Princeton Companion to to Mathematics . Princeton University Press. ISBN   978-0-691-11880-2 . http://books.google.co.uk/books?id=ZOfUsvemJDMC&pg=PA332 .   Porphyry ; Guthrie, K. S. (trans.) (1920). Life of Pythagoras . Alpine, New Jersey: Platonist Press . http://www.tertullian.org/fathers/porphyry_life_of_pythagoras_02_text.htm .   Guthrie, Kenneth Sylvan (1987). The Pythagorean