Dan Davis, On a tiling scheme from M. C. Escher, Electronic Journal of Combinatorics 4 (no. 2) 1997, #R23.
Doris Schattschneider, Visions of Symmetry, Notebooks, Periodic Drawings, and Related Work of M. C. Escher, W. H. Freeman, New York, 1990.
Doris Schattschneider, Escher’s combinatorial patterns, Electronic Journal of Combinatorics 4 (no. 2) 1997, #R17.
Stan Wagon, Mathematica in Action, second edition, Springer/TELOS, New York, to appear.
Stan Wagon, The Mathematica Explorer, Wolfram Media, Inc., Champaign, Illinois, to appear.