1. Bruno Ernst, The Magic Mirror of M.C. Escher. New York: Balantine, 1976.
2. Tilings and Patterns;Grünbaum,1987
3. P. Raedschelders, Private communication.
4. Private communication from Chaim Goodman-Strauss; also Pentakoch, Ptolemy mathcard no. 5, Ptolemy mathcard Co. 2000.
5. Robert W. Fathauer, Proceedings of the M.C. Escher Centennial Congress, Rome and Ravello, Italy, June 24–28, 1998. Germany: Springer Verlag, 2001. In press.