* John H. Conway, Heidi Burgiel, Chaim Goodman-Strass, "The Symmetries of Things" 2008, ISBN 978-1-56881-220-5 (Chapter 26. pp.
* John H. Conway, Heidi Burgiel, Chaim Goodman-Strass, "The Symmetries of Things" 2008, ISBN 978-1-56881-220-5 (Chapter 26. pp.