1. Carl Boyer, A History of Mathematics, 2nd edition, revised by Uta C. Merzbach, Wiley, New York, NY, 1991.
2. David M. Burton, The History of Mathematics, Allyn and Bacon, Newton, MA, 1985, pp. 122–123.
3. The Pentagram and the Discovery of an Irrational Number
4. I. N. Herstein, Topics in Algebra, Ginn and Company, Waltham, MA, 1964.